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For plot purposes I replaced an NPC with a doppelganger (one of the advanced "rumored" kind that can steal memories and skills). As this quest progressed, I found that it was actually rather difficult to detect a doppelganger. In this case, he was a cleric in an order of paladins & clerics, so detect magic was out of the question. Supernatural abilities can't be dispelled. And since the doppelganger had memories and skills (and, for plot purposes, had forgotten he was a doppelganger), a goof up or mistake in "performance" was unlikely. The party got through it and figured it out, but it did have us wondering:

Is there a spell or ability or some obvious racial "tell" that allows players to detect a doppelganger?

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Just curiosity: Is this a homebrew order of paladins and clerics that forbids casting detect magic or a published order? – Hey I Can Chan Feb 22 at 7:06
    
@HeyICanChan My reasoning was more "Magic magic everywhere!" I figured it'd be kind of like trying to detect water in the middle of the ocean. – Thomas Feb 26 at 4:53
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Nothing about a disguised doppelganger clues in adventurers that the creature's actually a doppelganger; such a tell would defeat the creature's purpose. However, even low-level adventurers have access to spells and special abilities that can detect a doppelganger.

The spell detect magic

The question says the spell detect magic is unavailable; I've included it here for completeness.

A doppelganger's change shape ability is a supernatural ability, which, despite not being subject to dispel magic, remains subject to detect magic. The caster of detect magic can identify a spell effect in place by making a Knowledge (arcana) skill check (DC 20 + spell level, in this case likely DC 22 for the doppelganger's change shape (alter self) effect). The spell alter self is dismissible, and a creature's refusal to dismiss an alter self effect because—I don't know—the creature says the effect's concealing unsightly scars from monster-fighting or compensating for a bad hair day or whatever is unlikely to carry weight for very long if the party suspects doppelgangers are about.

Another spell of the polymorph subschool

The GM may determine that discerning a doppelganger's presence using detect magic is just too easy, ruling perhaps that supernatural abilities can't be discerned by detect magic or that Knowledge (arcana) checks can't be used to determine supernatural abilities in place. (These decisions have no small long-term campaign impact, by the way.) In such a case, because the supernatural ability change shape "functions as a polymorph spell," the change shape ability is subject to the rules for the transmutation subschool polymorph, which says

You can only be affected by one polymorph spell at a time. If a new polymorph spell is cast on you (or you activate a polymorph effect, such as wild shape), you can decide whether or not to allow it to affect you, taking the place of the old spell.

Hence, the next easiest way for, for example, a cleric to detect a doppelganger is casting upon the suspected doppelganger the spell face of the devourer, a level 1 transmutation spell of the polymorph subschool, maybe followed by the spell detect magic and a successful Knowledge (arcana) skill check (DC 21) to determine a spell in place to confirm the face spell worked (although this should be obvious). However, identifying a spell in place can take as long as 3 rounds if the GM says the caster must wait until the third round of detect magic instead of being able to do so during the first. Also keep in mind that the caster knows if the spell's target fails the saving throw against the caster's spell. If the target's face does not go all Cthulhu and the target also didn't fail its saving throw, the target then has another polymorph effect that it's chosen to keep. Unless the creature has some plausible explanation for this (which it totally could—magic is weird), a doppelganger is a good (but not certain) bet.

The extraordinary ability scent

What I suspect most commoners do to avoid doppelgangers infiltrating their thorps and hamlets is train animals. Using the skill Handle Animal to teach an animal the trick detect—or pushing the animal to perform the trick untrained—should alert the handler of an animal that possesses the extraordinary ability scent to presence of something out of place. The scent ability allows the creature to "identify familiar odors just as humans do familiar sights," and the descriptions of the doppelganger, the supernatural ability change shape, and the spell alter self mention nothing about changing the doppelganger's smell. A loyal dog should alert its master to a doppelganger; a cat could alert its staff, too, if it felt like it. So could an orc or half-orc with the feat Keen Scent alert the party.

This is likely campaign-dependent. A GM may simply rule that changing appearance also changes smell. However, commoners in such campaigns remain at the mercy of the many, many creatures that can change shape.

Higher-level spells

Failing all this, the PCs could use the 2nd-level Clr spell reveal true shape, which doesn't change the doppelganger into its true form but layers a translucent illusion of its true form over its false one, leaving ignorant masses unfamiliar with magic perhaps doubting the caster's veracity. Nonetheless, the spell lets the PCs know the affected creature's a doppelganger, and that's the important thing. However, the spell reveal true shape is from an OGL adventure path (that is, the spell was originally for Dungeons and Dragons 3.5) and may be unavailable, making the 3rd-level Pathfinder-exclusive Sor/Wiz spell pierce disguise necessary instead.

Of course, the spell true seeing and everything that grants a like effect (like the navigator's eye) reveals a doppelganger, as will careful use of some let's-ask-the-gods spells (like commune and contact other plane).

"Seems too easy to detect a doppelganger then. How do they survive?"

The a-doppelganger-replaces-a-PC plot has been around as long as the doppelganger itself.1 Since most PCs don't go around suspecting their party members of having been replaced, the same hoary old strategy that's been around since 1975 still works today: pick off an isolated adventurer, replace the adventurer in the party, betray the party or lead it into traps, swoop remaining gear, repeat. This can happen anywhere, but it's most common when the adventurers themselves are isolated, like in a dungeon or wilderness.

In a large urban area, going around casting detect magic is probably frowned upon if not a cause for alarm, and fewer animals accompanying citizens means a decreased chance of accidental exposure. Further, a doppelganger can make city folks disappear easier because there's the much larger population to draw from. A doppelganger could live undetected its whole existence in the slums, the authorities blissfully unaware. An ambitious doppelganger, however, still must watch its step.

A revealed doppelganger isn't likely to be much more of a physical threat than any other monster, and maybe even less. What makes a doppelganger dangerous is all of the plans it's made and things it's done and gear it's grabbed and authority it's garnered before the PCs come along.


1 In fact, doppelgangers were so ubiquitous in adventure submissions that during the Dungeons and Dragons 3.5 era Dungeon magazine submissions guidelines forbade as a plot a doppelganger replaces an important figure. (Also forbidden were, for example, destroy an evil artifact and rescue a kidnapped princess.)

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Absolutely wonderful answer. Very thorough and well structured. – Thomas Feb 21 at 18:55
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One more method for detecting doppelgangers: another doppelganger. Presumably they must recognize their own kind if they reproduce as other humanoids do. Of course the doppelganger being used as a "doppelganger detector" may want to be concealed if she wishes to avoid being discovered herself by the revealed doppelganger. – RobertF Feb 21 at 22:34

In the case of a doppelganger posing as a spellcaster: just politely ask them to cast a spell for you. Even a doppelganger which copies "extraordinary and supernatural abilities" doesn't get to copy spellcasting.

More generally, if you think someone might be secretly evil, you might try using augury to test the results from killing the person claiming to be them. If the spell tells you: "hey, killing that NPC would actually be a really good idea!", that might not actually justify killing them but at least it means more pointed inquiry should be worthwhile.

The true seeing spell says that it lets you "see the true form of polymorphed, changed, or transmuted things" which would include doppelgangers.

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Spell: True Seeing

The subject sees through normal and magical darkness, notices secret doors hidden by magic, sees the exact locations of creatures or objects under blur or displacement effects, sees invisible creatures or objects normally, sees through illusions, and sees the true form of polymorphed, changed, or transmuted things. [emphasis:bold]

Doppelgangers have Shapechanger as a subtype, and "change" in an alternate form, and would therefore be revealed as a Doppelganger when subjected to True Seeing.


Food for Thought: Roleplaying

  • Direct questioning under the effects of Zone of Truth.
  • Active use of Detect Thoughts. Doppelgangers probably are thinking different things.
  • Doppelgangers have an at-will Detect Thoughts. That is a specific spell that is detectable by Detect Magic. Detecting that emanation could spark dialogue.
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While this answer doesn't draw on Pathfinder sources, I find that usually checking other editions can be pretty enlightening.

2e AD&D's Greater Doppelgangers, which can be found in TSR's Monstrous Compendium Annual Volume 2 (p.43), occupy the same role as the "rumored" advanced doppelgangers you speak of. They are indistinguishable from victims when disguised, can steal memories, etc.

Disguised greater doppelgangers can only be revealed by use of the true seeing spell or equivalent; their mental and physical disguises are even able to fool most psionics.

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This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – Ruut Feb 21 at 9:27
    
... It does answer the question. True seeing is the spell being asked for in the question. – mech Feb 21 at 9:43
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Pathfinder is the edition, not AD&D 2nd edition. Inspiration doesn't answer a specific question about a specific product. – Ruut Feb 21 at 10:25

Assuming that the Doppelganger was of the standard Chaotic Neutral variety, See Alignment, Detect Chaos, etc would made the deception obvious in the case of a Paladin, who must be Lawful Good to actually be a proper Paladin. While the Doppelganger may have stolen the "Aura of Good" ability, Paladins do not have a corresponding aura of law as a class feature, and the Doppelganger's inherent chaos would show through to these spells even if it did. Furthermore, routine alignment checks are actually fairly likely to take place in a Paladin-and-Lawful-Good-Cleric-Only enclave, where privacy concerns on the matter are likely to be minimal and the required 1st-level spellcasting is nearly omnipresent.

If the Doppelganger has managed to conceal it's alignment or adopted the actual ethos of its new form over time or something, then the matter is trickier:

Assuming the Paladin or Cleric impersonated is not him or herself a Monstrous Humanoid, the Monstrous Humanoid favored enemy choice and similar effects (like the Bane inquisitor class feature) will proc on a Doppelganger, while a Ranger with favored enemy Humanoid(Human) (or whatever) would note that its abilities fail to activate on the impersonated target.

Regardless of what it does, a Pathfinder Doppelganger is immune to all polymorph effects while shapechanged but cannot benefit from such effects without reverting to its true form. Each time it is subject to a new polymorph effect (for example, because it went to a meeting and cast Aspect of the Nightingale out of habit) it is prompted to choose between a mysterious currently ongoing polymorph effect and the new one it just tried to cast. Having no memory of changing shape, the Doppelganger has no reason to trust the mysterious preexisting polymorph effect on its person and would probably accept the new effect in its place, causing it to revert to its true form.

Somewhat fundamentally, magical impersonation can be overcome with sufficiently high Perception scores, possibly augmented by the use of True Seeing. High Perception scores can be easily achieved in Pathfinder through varied means, but one particularly effective method is the use of Hawk Clan variant of the spell Summon Totem Creature for hawk familiars, which can provide an untyped self-stacking (1d3+1)X3 bonus to perception per casting. The average Doppelganger has an average disguise check of 39, which is well within the average of +54 Perception achieved by 6 unaugmented castings of the spell.

An antimagic field, and similar effects, is also capable of ending the disguise without any sort of roll being involved.

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