In most situations where a femdom slave is involved in BDSM there are safe words that are used. In some situations where the submissive slave has their mouth gagged or they are incapable of speaking, non verbal signs are used instead of safe words. These non verbal signals can be a variety of things but some of the most familiar are the clenching or unclenching of one or both fists, snapping of their fingers, dropping a ball, bell, or other safe instrument, or letting out three loud grunts in succession.
Whatever safe words or signals you choose it is vital that they are adhered to. It has been known that some dominants may ignore safe words or signs and if you get a reputation for this you will find it very hard to find submissive partners. In some situations there are no safe words used and this normally happens when the submissive has full faith in the dominant and this is quite common among husband and wives.