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As a player (or DM) might use a familiar as a spy, are there ways, magical or otherwise to detect that a cat is a familiar, or just a house cat?

I've read the answer to Are familiars considered magical for effects like detect magic? and agree with the accepted answer that they are not. So what might be other methods for exposing the nature of the creature in front of you?

And I mean short of killing it to see if it disappears ;)

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  • \$\begingroup\$ Note that the question you linked is asking about a different game/edition that also happens to have a Find Familiar spell, so any answers there have no bearing on how Find Familiar works in D&D 5e. \$\endgroup\$ – Ryan C. Thompson Feb 19 at 15:05
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    \$\begingroup\$ Here is the relevat D&D 5e question: Which features of a wizard's familiar, if any, are considered magical? \$\endgroup\$ – Thomas Markov Feb 19 at 15:08
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    \$\begingroup\$ Is it considered gauche to ask? \$\endgroup\$ – NautArch Feb 19 at 17:37
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Detect Evil and Good

The spell detect evil and good will allow you to determine if a beast has a creature type consistent with a familiar:

For the duration, you know if there is an aberration, celestial, elemental, fey, fiend, or undead within 30 feet of you, as well as where the creature is located.

This works, since find familiar states:

Appearing in an unoccupied space within range, the familiar has the statistics of the chosen form, though it is a celestial, fey, or fiend (your choice) instead of a beast.

This won't explicitly tell you "this creature is a familiar", but if you know anything about familiars, you will know that a celestial, fiend, or fey appearing as a harmless beast will be rather suspicious; suspicious enough to say "this is probably a familiar".

Locate Animals or Plants

The spell locate animals or plants can tell us what need to know:

Describe or name a specific kind of beast or plant. Concentrating on the voice of nature in your surroundings, you learn the direction and distance to the closest creature or plant of that kind within 5 miles, if any are present.

With this one, if you suspect a nearby cat may be a familiar, cast this spell and concentrate on cats. If the suspicious cat pings with the spell, you know it isn't a familiar.

Channel Divinity: Arcane Abjuration/Abjure the Extraplanar (require a saving a throw)

These are conditioned on the suspected familiar failing a wisdom saving throw. The Arcana Cleric's Channel Divinity Arcane Abjuration says:

As an action, you present your holy symbol, and one celestial, elemental, fey, or fiend of your choice that is within 30 feet of you must make a Wisdom saving throw, provided that the creature can see or hear you. If the creature fails its saving throw, it is turned for 1 minute or until it takes any damage.

The Oath of the Watchers Paladin's Abjure the Extraplanar ability reads:

You can use your Channel Divinity to castigate unworldly beings. As an action, you present your holy symbol and each aberration, celestial, elemental, fey, or fiend within 30 feet of you that can hear you must make a Wisdom saving throw. On a failed save, the creature is turned for 1 minute or until it takes damage.

These aren't totally reliable, as they require a saving throw.

66% of the time, it works every time.

There are a few options for detecting fiends and celestials that do not detect fey. These include:

  • The Paladin's Divine Sense feature.
  • Helm of the Gods (from Mythic Odysseys of Theros)

Additionally, the Oath of Ancients Paladin's Turn the Faithless channel divinity turns a fiend or fey on a failed saving throw, but has no effect on celestials. The Oath of Devotion Paladin's Channel Divinity turns only fiends.

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    \$\begingroup\$ @DarthPseudonym There were a couple of other options that worked for celestial and fiend that missed fey. 66% of the time it works every time! \$\endgroup\$ – Thomas Markov Feb 19 at 16:15
  • \$\begingroup\$ Great option. It might be worth adding that if they don't have access to this spell, the higher level Locate Animals or Plants could similarly help. \$\endgroup\$ – Gandalfmeansme Feb 19 at 17:26
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    \$\begingroup\$ @BenBarden The cat not being a beast makes it an inelligible target for the spell since the spell finds beasts and plants, not fey, celestials, or fiends. \$\endgroup\$ – Thomas Markov Feb 19 at 17:49
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    \$\begingroup\$ @ThomasMarkov but the spell text does not say that it only finds beasts and plants. It says that it finds creatures and plants that are of the given kind. So, for example, if you somehow had a plant that was a cat, and you searched for cat, it seems like you'd find it. I admit that there's at least an implication that it would skip over non-beast cats, but I assert that it's not entirely clear, and could be reasonably interpreted either way. \$\endgroup\$ – Ben Barden Feb 19 at 17:52
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    \$\begingroup\$ A DM who has introduced a real small beast might have it reacting by running away anyway if abjured - I don't imagine the "presenting" involved is supposed to be done in a way that would otherwise be inviting or playful. Although there could also be comedy value in having the cleric burning with holy fire, yelling "away foul creature of the abyss" and having whatever creature approach and sniff the holy symbol to check whether it is some kind of treat :-). \$\endgroup\$ – Neil Slater Feb 20 at 11:35

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