So many other comments about reading facial expression. No one has mentioned reading body language. I, for one, tense up in different ways during a scene. My hands are very telling also. The sounds that are made can tell tales as well. (even gagged, subs can make noises to let their Dom/me know how close they are to reaching their limit....when to slow down, speed up, rest a bit, or even stop.
As to the Dom/me wearing a hood or mask....hmmm, yummy. I hadn't thought much on that but it does add a sinister air to him/her. My thanks to Snow for mentioning it.