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This
strip was featured on our facebook page recently and it was labeled as Something Only They Would Say.
The Cyclops/Wolverine thing is also the page quote.
What if it's not done with dialog, like if a notoriously clumsy character knocks over a whole bunch of fragile things?
Well, there's No One Else Is That Dumb, which lists several non-verbal examples.
Also Tropes Are Flexible; "something only they would do" could fit under the heading- say, the Big Eater gets distracted from the entire Spot the Impostor situation by a nearby vending machine, thus proving their identity.
It can also be Imposter Forgot One Detail if it's something they aren't doing that gives them away, and Impostor-Exposing Test if its something like "the real Bob can chug a bottle of Blazing Inferno Hellfire Sauce"
Edited by Scorpion451
Some characters are unsure if a particular person is someone they know, and are convinced of their identity when the person display some annoying habit or trait they know that person to possess. Often like this:
Or in the first X-Men movie, when they think Wolverine might be Mystique in disguise, so he insults Cyclops to prove his identity.
Edited by shigmiya64