A twist cheats when the only reason you didn't see it coming is that the movie hid something from you.

That's the whole test. Withheld information isn't a twist. It's a trick, and an audience feels the difference even when they can't name it. A real twist uses what you already had and shows you that you read it wrong. Nothing new arrives. Your understanding changes.

Rian Johnson does this in Knives Out by giving away the death in the first act. You think you know exactly what happened and who's covering it up. He never lies to you. He lets you build the wrong story out of true pieces, then rearranges the pieces. The second watch is better instead of worse, which is the proof.

So write the twist first and work backward. Every plant has to do a job in the moment it shows up, or the reader files it as a clue and waits. A detail that only exists to pay off later reads exactly like a detail that only exists to pay off later, and readers catch it by page twelve. The good ones look like they're about something else.

Then run the honest check. Watch your own movie as somebody who knows the ending, and ask if the first hour still holds. If a character does something they'd never do unless the script needed them to, that's the cheat. Fix the behavior, not the reveal.