just (you) watch
You will see that what I have told you or predicted will turn out to be true eventually. I'm going to be a huge success some day, just you watch! Just watch, I bet Susan will walk through that door any minute now to apologize.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
(You) (just) watch!
Inf. Just pay attention to what I do, and you will see that what I said is true! Rachel: I'll get her to change! You just watch! Andy: Good luck! Andy: You watch! You'll see I'm right. Sally: Sure, you are. Bob: Watch! This is the way it's done. Bill: You don't know what you're doing. Bob: Just watch!
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.