Within many of them you'll find answers specific to the various gals you may be seeing.

Don't forget that if you're seeing one of the good providers you're generally dealing with a professional. If you are feeling "vanilla" as you say (which I take to mean pretty standard activities and positions) you could simply tell the lady you feel a bit inexperienced with all that sex play has to offer and she will probably have more than a few suggestions to offer.

Some of this is derivative of mrfisher's suggestion to read the reviews to know what to expect (and maybe how to shift the review-expected to YOUR expected).  
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E.g., "... after the cleanup, we were lying there, chatting. The next thing I know, she's stroking me back to attention and asking if I'd like another try." = She'll take charge for you.  Another e.g., "... after the cleanup, she went to the desk and started checking her phone. I spent the remaining 40 minutes counting the dots on the ceiling tiles. Maybe I should have said something?" Well, yeah!  And another e.g., "... after the cleanup, she went to the desk and started checking her phone. I said, "We've still got 40 minutes. How about we keep going?" ... and so on from the Juicy Details (VIP Only) portion of the reviews.

1) Asking her to show off for me, before we dive in. That provides all sorts of clues.
2) Asking myself what I really want from her and what might be available during the encounter.  
I used to be pretty passive, waiting for clues or invitation, but now I ask the questions (verbally or physically) as the session develops. Timing is important. Boundaries tend to change as things develop.