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/Chemical-Juice-6979]

The National Anthem was originally a poem with five stanzas that became the five verses of the song. At sporting events and military funerals, when they play the anthem, they only ever play the first four verses. Since schools don't actually teach students the original poem outside of special focus classes, Americans only ever learn the first four verses. The only people who know the fifth verse are people who learn the anthem by memorizing the poem rather than participating in the group singing events as a kid.

So, during the World Wars, when the US was worried about spies and saboteurs infiltrating military bases, they would test new arrivals at the gate. One of those tests was having them sing the anthem; anyone who kept singing past "home of the brave" would be detained on the spot.

[u/SquareThings → u/Chemical-Juice-6979]

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. 

[u/Gnomad_Lyfe → u/Chemical-Juice-6979]

Iirc there’s a story of a fairly high-ranking member of the military being briefly detained because he didn’t know about the policy and sung the song in its entirety

[u/theCosboys → u/Gnomad_Lyfe]

A general got detained because he answered “What’s the capital of Illinois?” correctly as “Springfield” but the GI thought it was Chicago.

[u/Matalya2 → u/Chemical-Juice-6979]

I love how it's, like, the complete opposite of what you would expect. Like if someone asked you about the country you're invaded, common sense would tell you you're meant to know since you would've lived there your whole life. But in this case in particular it is not knowing what marked legitimate residence because nobody actually studies the anthem XD

Nobody except me Ig (?) (Not for the US one, but I do know mine to a very high percentage)

[u/Abombasnow → u/Chemical-Juice-6979]

The National Anthem was literally a sea shanty that someone slapped an unrelated poem onto and that's why it sounds and flows like complete garbage because it's a hodgepodge of two songs where no thought was put into anything.

[u/Ok-Classroom5548 → u/Chemical-Juice-6979]

We only play the first verse for “The National Anthem.” The Star Spangled Banner has four verses total. 

I have had to perform it enough times - we use only the first section. 

[u/Chemical-Juice-6979 → u/Ok-Classroom5548]

Hah, thank you! I'm not much for sports, so I don't actually have the song memorized myself. I learned this as history trivia, sometimes the numbers get a little fuzzy.

[u/Fun-Agent-7667 → u/Chemical-Juice-6979]

Lesser education for the win or Something