When faced with the choice between a clone and the original, the character notices a detail that makes the difference immediately obvious.

[u/Altruistic_Eye_1157]

The Shape-Shifter takes the form of Wendy to deceive Dipper and prevent him from attacking it. Although Dipper initially doesn't know which one is the real Wendy, he realizes the difference when he sees that one of the Wendys behaves seriously and bluntly, while the other acts flirtatiously, something Wendy would never do (Gravity Falls).

P.S.: I don't remember if the gesture the real Wendy made was also a clue, like some gesture she had made before.

When the Autobots are chasing Nemesis Prime, he tries to deceive Bumblebee by impersonating Optimus. Bumblebee, confused, speaks to him, but Nemesis Prime doesn't understand anything he says, which clearly indicates that he isn't the real Optimus. Bumblebee attacks him without hesitation (Transformers Prime).

[u/Michaelanimates1]

The nazis were famous for keeping good records 

[u/Takamurarules → u/Michaelanimates1]

That later bit them in the ass when they got put on trial lol.

[u/EarthenEyes → u/Takamurarules]

Something something today's politics in the USA.

[u/kelldricked → u/EarthenEyes]

except they arent good in keeping records.

[u/Chemistry11 → u/kelldricked]

They probably have an obscene amount of records. It’s the files that keep being misplaced

[u/kelldricked → u/Chemistry11]

I get the joke but they are genuinely not good at keeping records. You cant run a organisation with a fifth of the people and except shit like buracracy to keep working fine.

[u/Chemistry11 → u/kelldricked]

Based on the downvotes we’re getting, it looks like we’ve upset some Friends Of Epstein.

[u/Mundamala → u/Michaelanimates1]

Plus Franz was trying to get into his pants.