Body language experts will tell you a hand to mouth or nose gesture is an unconscious attempt to cover a lie.
The key is to listen to what he said immediately prior to this gesture.
LOL...posted at exactly the same time. I think we have a winner ;-)
I know someone who has a habit of covering his mouth about half the time he is speaking, and it couldn't possibly have anything to do with his lying. He does this in the midst of discussions mostly about things academic, and not in the course of pretending to be an expert about any of it. I find him careful to separate fact from opinion during these discussions.
ML/NJ
Notic that he blinks after a lie too.