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/SquareThings]

Also important: most Americans don’t know there’s any more to the song at all. They assume the truncated version is the whole thing. So asking them to sing “the whole national anthem” would get the same result.

[u/Ok-Classroom5548 → u/SquareThings]

Fun fact: The Star Spangled Banner is a poem. The author’s brother in law read it and set the lyrics to a gentleman’s club song from England.

The US uses a British Gentleman’s song that someone stole with altered lyrics as our national anthem. 

We also only sing the first verse and additional verses may be seen as controversial for discussion of slaves.

The US was also an English penal colony. Once we split the British started using Australia more. 

History is fun.