Like any other "trick" play, enforce the letter of the rule here. If you can see any head bob, so can the defense, and it constitutes simulating the start of a play and thus a FS.
If I see a slight head bob and the defense does NOT encroach, I will speak to the QB about it. If they do encroach, it's a FS.
Some officials are stricter about this: a FS is a FS, whether or not the defense encroaches. For me, this approach undercuts the opportunity to warn in borderline cases and thereby improve the flow of the game.