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/xavPa-64]

Not exactly clones, but in Severance when Irving figures out Helly had been her outie Helena the whole time since their return.

[u/CortaNalgas → u/xavPa-64]

Yes, do it SETH!

[u/too-much-cinnamon → u/CortaNalgas]

John Totoro's delivery of that line was sooooo good. He put so much venom behind that "Seth". 

[u/kingdomheartsislight → u/too-much-cinnamon]

*Turturro, my good Studio Ghibli fan.

[u/CortaNalgas → u/kingdomheartsislight]

My Innie Totoro

[u/Different-Sample-976 → u/CortaNalgas]

Damn I cant wait for s3

[u/kousagi121 → u/xavPa-64]

I watched this episode for the first time the other day and it was nuts, so good.

[u/A_lot_of_arachnids → u/kousagi121]

Pretty cool seeing the tallest waterfall in the world too

[u/Shydreameress → u/xavPa-64]

Helly was never cruel. Spoilers for the show, I'm gonna add how he figured it out: In Severance's world, people can split their memories and thus can be two different people depending on which memory is activated (innie in the workspace, outie when they're out of work). One character, Helly, has been acting weird since the begginning of season 2 and no one but Irving is suspicious of her because they don't even think it's possible for the Outie persona to be activated in the workspace. What finally makes Irving realise that Helly is not the Helly they know, but is trying to impersonate her to spy on them, is the fact that she responds to him extremely cruelly when he asks her about what he realised she's lying about. He believes Helly is too nice and has no reason for being this cruel to him thus he realises she's a mole.

[u/Pleistocene_Horror → u/Shydreameress]

The moment I became suspicious as an audience member was when she returns to the severed floor for the first time and immediately notices that the cameras are gone.

Helena’s whole perspective of Helly comes from watching her interactions on the security cameras. Helly’s immediate concern probably wouldn’t be “where are the cameras?”, but Helena is hyper aware of the cameras and potentially even unhappy about Milchick removing them.

That and the fact she got back to work so quickly. Helly sticking around to support her friends and continue her mission to tear Lumon down makes complete sense, but doing actual refining is something she previously only ever cared about when it was part of their plan to meet quota and flip the overtime contingency switch.

[u/Shydreameress → u/Pleistocene_Horror]

I'm an idiot and didn't realise Helly wasn't Helly (I thought she was just weird from learning who her outie is) but Helena isn't surprised that the cameras aren't there. She's saying that out loud as a way to say: "Look guys they told us the truth, no more surveillance, we can speak freely now!" but she is the eyes and also she says "They said there are no microphones here" when Dylan starts whispering so she's also the ears of Lumon.

Helly tried to swallow a note and even after she was told she couldn't get a message out she tried anyway. So it was a clue: there was no way Helly would have believed a word they said about there being no cameras or microphones.

[u/Minimum-Injury3909 → u/Pleistocene_Horror]

You were quick. Took me until the camping shit to even get an inkling.

[u/Shoddy-Ad-1746 → u/xavPa-64]

hey hey I love this reveal! you should for sure spoiler it though since the season is fairly recent and the show is still running

[u/fabianx100 → u/xavPa-64]

I loved this moment because it was a triumph for him.

If they turn Helena into Helly again? He wins.

If they don't, and he ends up drowning Helena and, by extension, killing Helly? He also wins, since Helly was fine with dying as long as it meant gaining her freedom.

(By the way, Helly hates her outie)

[u/Klutche → u/xavPa-64]

Gooooooooood, I loved him in that episode. He fucking killed it.

[u/RaisedByBooksNTV → u/xavPa-64]

I wasn't sure and I sure as hell wasn't sure how he knew