Another suggestion is to watch the opponents breathing. If they take small shallow breaths, You need to do some reconnaissance work before you can perfect the art of catching the breathing bluff. Study each opponent when they make a call. Watch how they breath and make a note of how they breath when they make a call, bet and are strong on the river. Regardless if they win, was there hand strong? If so how did they breath while waiting for the call/raise. Bank this information. Next, study them when they bluff and muck. Watch what they do when they raise someone all in and during that period while they wait for the call/fold.. how do they breath. Once you have seen how they react you will know what to do when faced with action toward your hand. One of the easiest tells
to spot in poker is the holding in and forcing shallow breaths. If you notice your opponent tensing up and controlling his breathing, he is weak or bluffing/fearful of a call. of course this does not always work however body language and the art of studying behavior is the best way to spot a bluff and the only practical way to crack an opponent live IMO.
There is a video of Scott Siever where he bluffs and covers his mouth against Daniel Negraneu(spelling) when asked later why he covers his mouth he admits its to prevent opponents from monitoring his breathing habits.