'Fold' - "Are you Sure"?
 

Don’t fold instantly unless you’re sure - By Mike Caro :brains:

Sometimes you can get a tell on an opponent simply by not folding too quickly. I’m not telling you to slow up the game, but occasionally — when you’re in doubt about whether to call — you should conspicuously study your opponent.
Scrutiny

This extra scrutiny will sometimes make a player who’s bluffing uncomfortable enough to give you the clue you’re seeking. You just have to wait a few seconds and observe.

In general, if the player remains relaxed, you should fold, as you originally intended. But if there seems to be growing tension in the opponent and he becomes totally “poker faced” and motionless, even not breathing, you should consider calling.
Fear

Remember, a player who is bluffing will usually do nothing unusual for fear of triggering your call. It’s the absence of animation and the suspenseful tension that let you know that an opponent is more likely than usual to be bluffing. When you’re in doubt and fold too quickly, you often lose the opportunity to capitalize on this powerful tell. — MC ;)