Other than viewing her profile on-line, I am wondering when is the right time to ask what she will and won't do? Or should I just let her lead the way?

and never ask before you meet, for you may scare her off. That is what reviews are for

If a specific activity is very important to you I would not recommend booking with a lady unless you see that activity mentioned in her reviews and mentioned consistently. This is also where the rep of the reviewer becomes important. I've seen guys with only a few reviews lie about specific activities often-usually because the review is fake.
Asking about specific activities once the appointment has begun is usually OK but it's a bit late in the game if you wanted something specific that is not on her menu.
As an aside. I'm fairly easy to please. I'd much rather spend an hour or two in a good lady's comfort zone than keep bugging her to do things she doesn't want to do. To a certain extent, I view any appointment with a new lady as "exploratory". Based on how things go I may book again or I may not but I'm going to try and make the best of the time we spend together. If she is a reputable provider she will do the same regardless of her limits.

If the reviews don't help or if she's new, just ask her. I just bring it up as part of the "getting to know you" conversation. I have no problem talking to people and putting them at ease. Usually, we are to the point where we are on the bed and I'll just say that I really want her to be comfortable. "So,is there anything that is off limits or that you really do not like to do?" She will readily tell you and appreciate that you asked. It prevents unpleasant disruptions later. Sometimes, she will say that pretty much anything goes. Then I may ask if CIM, Greek, whatever, is ok? Sometimes she'll say "anything except for...

I've never had a negative response to asking.