Law 42 – Dummy is entitled to 
(a) ask declarer (but not a defender), when he has failed to follow suit, whether he has a card of the suit led.
(b) try to prevent any irregularity **by declarer.
(c) draw attention to any irregularity, but only after play is concluded.

** Note: He may, for example, warn declarer against leading from the wrong hand.

 All players except dummy may ask for a review of the auction.

 Do not pull up a card from your hand, push it back, and then pull up another card. Instead, wait until you are sure of what card you are going to play before you pull it up. Pulling up a card you don’t play can be construed as giving your partner unauthorized information.

 Once the cards in a trick have been turned over, no one can ask to see the cards.

 Avoid hesitations in your play, which can be construed as giving unauthorized information to your partner.

 Play briskly. Slow play is subject to penalty.

 Above all, avoid any behavior that would make anyone (especially newer players) feel uncomfortable.