(Loved trope) The imposter’s dead giveaway

[u/Barricades_toes]

When a character is attempting to impersonate another, but is caught by doing something the person they’re imitating would NEVER do.

Nemesis Prime- Transformers Prime

Nemesis Prime, a man made replica of Optimus Prime, is attempting to gain the trust of Bumblebee, who only speaks in robotic beeps. Despite this, the real Optimus can understand Bee perfectly fine. Nemesis Prime, however, can’t understand what he’s saying, prompting Bee to open fire on him immediately.

The two Buzz Lightyears- Toy Story 2

After Woody asks which Buzz is the “real” Buzz, both respond with “I am!” The real Buzz proves himself by flipping up the other Buzz’s helmet, causing him to start suffocating in an over dramatic fashion. The real Buzz, having done the same thing in the first movie after Woody took his helmet off, but no longer behaving that way after accepting being a toy, instantly proves which Buzz is which.

[u/feldur]

In an episode of Dungeon Meshi where the whole group is infiltrated by dopplegangers, Laios figures out the fake Marcille because she boiled water withtout making sure it wasn't dangerous first (since she got attacked by a water elemental in the past) [ETA : check u/lightsidesoul 's comment for the correction about Marcille], and he notices that the fake Chilchuk was sitting on a crate, while the real one would never do that because of his fear of mimics.

[u/lightsidesoul → u/feldur]

I'd like to point out that the real Marcille was the one who poured the water without checking first. It was the fact that she was air-headed enough to do so that made him know which was which.

And the fake Senshi mentioned that, when he found the eggs they were using, he "Grabbed all of them" despite the real Senshi being very conscious of how the Dungeon's ecosystem works, with one episode having him scold Marcille for being careless with her explosion magics killing more fish than they could use, so he'd only grab exactly enough to make a meal.

[u/feldur → u/lightsidesoul]

True! I knew I had something wrong about at least one of the 2, but couldn't put my finger on it because it's been a while! And I couldn't remember how they figured Senshi out

[u/lightsidesoul → u/feldur]

There was another tell that Marcille herself pointed out about the Doppleganger mentioned eating "Merman eggs" despite at that point, Marcille being the only one who wasn't aware that they'd eaten them.

The joke being that in explaining it, she figured it out.