First, I'd ask for recommendations. Once you have some names, check a provider's TER profile, reviews, website, and social media content for clues about what kind of play they might have enjoyed in the past - and to see if you're attracted to them.

In your case, any mentions of butt play in a review, phrases such as "toys for boys" on a website, mentions of being kinky or fetish-friendly, writing steamy erotica focusing on butt plugs or strap-on play...

Then, get screened for your top choice.  

When setting up the session, ask the provider if it's okay to to talk about your personal preferences briefly on the phone after you've been screened, because you want to make sure this is the right fit. When you do discuss things on the phone, keep it very brief and professional.

Don't ever ask, "Can we do X, Y, Z in session?"  

Simply mention your preferences as part of sharing about yourself, and ask if she thinks you two will get along

Any other tips, ladies and gentlemen