i think it is the dom's responsibility to watch every nuance of hte sub- you should be able to tell if the sub is tensing after each blow, or bracing before the blow- all pain is going to cause some tensing, but if hte tense happens before the strike, it probally means that the previous blow was too painful for hte bottom. and dont take for granted that hte sub will even tell you to stop if they need you to- sometimes they are too far gone to let you knwo, and for some (lisa) it is a matter of pride that they try to take all you can give.

so every once in a while, talk to her. but dont say "are you ok?" and stuff like that- unless it is really warrented- waht i say in those cases is - "do you remember your safeword, girl?" that is a clue to the sub to think of their word, and if they are in a situatioin where they need it you remeind them to use it, without coddling them, and breaking the scene.