"I know how you did that!"
Now not everyone says it the same way. Variants include "I know how that's done" or "It's in his pocket/ other hand/ hat". And sometimes it doesn't actually mean "I know how that's done."
I guess my major complaint with people who have decided that they have sussed the secret is that they believe that if they figure out one part, they figured out the whole thing. For example, the lady sawed in half illusion. People just say "Oh, she's folded in half in there" and that's that. Even if that were true, that's just one step in the entire process. I feel like that would be like me eating Kentucky Fried Chicken, being able to deduce one of the spices used and then declaring that I know the secret recipe. There's a bit more to it.