well, to be honest, what i did with my (OL) Dom when we first met was to be 100% frank. I mean, I'm quite new to the lifestyle, as well, so he, if you don't want to see it as a Dom/sub, is more experienced in the whole situation. I told him about the fact that I'm not entirely sure what my limits are and how far I can take it. I am lucky because he understands what it's like to be new and uncertain. Hopefully your future Mistress (if things work out) will be equally understanding.
Still, my opinion on the matter is to be honest. I don't know you or her, but in the relationship I have with master right now, when I don't understand something, or when I'm really curious about something, I ask for permission to ask a question. However, as you're still in the beginning of the relationship, I think that both of you need time to get to know each other better--and I'm not only talking about fetishes or limits, I'm talking about getting to know the person behind the title--if that's what you both want.