An interesting question, my angel. I would put forward that it is a joint responsibilty as once in the throes of passion one is likely to overlook things-not maliciously but due to concentration being elsewhere! Clearly, should a sub be unable to clearly communicate then there is serious problem with the play as it should be understood that a signal must always be ready to be given if a potential limit is reached or a danger threatens.
A Dom/me must be acutely aware of his/her sub's state of mental, physical and spiritual wellbeing during times of limited communication as it is then the Dom/me's responsibilty to perceive the problem and act if the sub cannot do so in time.
A sub has a special responsibilty to be honest and unafraid to say no or point out a potential hazard.
It boils down to mutual trust and clear communication. The moment one feels that he/she should keep secrets and feels that the partner can be excluded from his/her confidence the problems begin.
The keys are and always have been trust and open communication.
Neither sub or Dom/me is above their counterpart, all are equal.
So-that's my opinion.

*hugs*

Ed.