On those occasions when you manage to bluff someone into telling you something by acting like you know more than you really do, and then he asks you, When did you find out?–resist the temptation to be the smirking A-Hole who says, Just now–when you told me! After that bush league play, he won’t tell you shit.
The best responses run along these lines: We’ve known almost since the beginning. Your opponent will spend the next few moments trying to figure out where he got careless and screwed up. Meanwhile, you can follow with, “But there’s just one or two things I don’t understand.” You’d be surprised at what he’ll tell you.