One of the first rules I set for my submissive was that he was to tell me whenever he found himself aroused by what we were talking about or doing. In doing so, I was able to learn, rather quickly, what sorts of fantasies he had, whether or not he felt comfortable offering them to me. It also gave him a sense for some the structure I would require.

Further, having him tell me he was aroused or hard by using protocol, i.e."I am hard now, Mistress" also acted a reminder concerning why we were discussing or doing the things we were doing.