About the card protector: it doesn't happen often, but if a player uses one AND she is sitting behind you AND she looks at her hand before your action AND does not put her card protector on her cards, then she won't play the hand. The converse is not necessarily true though.

Something you should watch for is the comfort state of your opponents. See how they act when not in a hand. Players who plan on folding usually act in the same way as when they have no cards at all. They have already mentally folded and their head is not in the game.