Well..this is an interesting question.
We are both new to this, and lets just say that the transition from being my usual independent smart ass self into the submissive pain slut that I know I really am has been challenging.
Master and i have been together for almost 15 years (married for 12), and I know him better than I know myself. However, I will be damned if I can pick up on his subtle "domly" cues quick enough.
Anyway, what brings out my submissive side is when master tells me to do something NOW, or calls me his slut, or just the change in the tone of his voice. Though as I stated above, these are hard to pick up on at times, cause master to become frustrated I am sure.