How can characters/players identify that a polymorphed dragon is a dragon?

Question

Many dragons (e.g. ancient gold dragons) can polymorph using the Change Shape action:

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).

In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

If a player or character wants to confirm whether a humanoid/beast is a polymorphed dragon, what is the nature of mechanical checks/spells they can carry out? Are there particular DCs for these checks? Do any classes or races, e.g. dragonborn, have an innate advantage or disadvantage?

Answer 1

True seeing, 6th-level spell

This spell gives truesight which allows to

perceive the original form of a shapechanger or a creature that is transformed by magic

without any check/DC.

Dragons don't have the shapechanger tag, but their Change shape ability does specify that it is magic :

Change Shape. The dragon magically polymorphs into etc.

Note that this method is costly (a 6th-level slot and 25gp per casting) and therefore can't be used to find all polymorphed dragons you meet. However if you suspect that someone is a dragon, it could work to verify your suspicion (finding situations where the PCs suspect, but aren't quite sure that someone is a dragon will be left as an exercise to the reader).

Witch sight, Warlock invocation

(thanks @BenBarden for catching this)

Prerequisite: 15th level

You can see the true form of any shapechanger or creature concealed by illusion or transmutation magic while the creature is within 30 feet of you and within line of sight.

The text from the Change Shape dragon ability doesn't explicitly say what kind of magic is used, but the text

The dragon magically polymorphs into etc.

hints at transmutation magic. A strict RAW reading makes this not work, but strict RAW is where madness begins.

Answer 2

The ability's description states that the dragon retains its mental attributes, indicating that whatever form he takes, he still thinks like a dragon. Somewhere in his speech, in his mannerism, or even just in the way he thinks, will be clues as to his true nature. How carefully he hides those clues should be reflected by a deception check, as the dragon is definitely trying to deceive the PCs regarding his true identity.

Mechanically speaking, I'd have the dragon roll a deception check against the PCs' passive insight score. As no one is likely to be looking for these clues initially, so it would make sense to grant advantage to the dragon, as well as disadvantage to the PCs (or -5 to their passive scores). If the players are actively looking for these clues, however, have them roll a regular insight check instead of using their passive score.

If one of the PCs is a ranger whose favored enemies are dragons, he should have advantage on the check, or +5 on his passive score, as he's well aware of dragons' general behaviour. If his favored enemies are humans (or whatever humanoid species the dragon is posing as), the ranger should also receive these benefits, but success wouldn't directly point to a dragon. In other words, the ranger's familiarity with humans allows him to detect that the creature in front of him is not a natural human, but not that it's actually a dragon in disguise.

As @Pierre Cathé's answer pointed out, the True Seeing spell would immediately reveal the shapechanger's identity. If the PCs are not high enough level to cast this spell, a Gem of Seeing would achieve the same results.

The 2nd level spell Moonbeam, available to druids and paladins, could potentially reveal the dragon. In principle, it would not, as the dragon does not have the "shapechanger" tag, but a lenient DM could allow it given the similarities between the dragon's shape change ability and that of "ordinary" shapechangers. Note that this spell requires a failed save to work, as well as dealing damage, so it's not necessarily advisable if you don't want to fight the dragon. The dragon can also use his legendary powers to succeed on the saving throw, but only a limited number of times. Moonbeam can be concentrated on and used multiple times on the target, meaning that eventually it would run out of legendary saves.