...is ask, and pay attention to the answer! Be open to hearing whatever your partner says without judgment, whether it's a spouse, lover, provider, or client. Listening to someone doesn't mean agreeing to actually do particular acts. But it can lead to more authentic connection. While the element of performance vs. reality varies, with providers I think it's very helpful to have conversations about what you want in a session. If you want a GFE fantasy where the provider is magically into everything that you're into when you're into it (within her limits), say so. If you want a provider to actually share what she might enjoy with you in the moment, ask if she'd be willing to do that.
Besides a conversation, nonverbal cues, flirting styles, talking dirty, what kind of porn someone likes, etc. are all hints as to what revs their engine. To get a hint about providers' desires beforehand, read their websites or social media accounts. If a provider's site is full of photos of herself in black tie attire sprawled across supercars, she might get off on being gifted & modeling upscale lingerie. Often ladies have a gifts section or wishlist that will give you hints as to what they might enjoy. Personally, I advertise "fetish friendly companionship" because BDSM is one of my passions.
Providing is a service so I follow your reasoning that sub providers might get off on it more--if they are service-oriented submissives. However from my perspective as a (mostly) sensual FemDom provider, I find it super hot to have men pay me for playing a role that I love, and take control of them for an hour or an evening. Money is a form of power, after all, and I kink on power exchange.
One special friend, in particular, has asked to bring My *actual* kinky fantasies to life, and offered himself as a vessel to do that - and it's been exhilarating! During our last session, I hired a hot bi stud for a threesome. Having the sexy male escort fuck me over My submissive friend's bound body as he lapped at my clit is a kinky memory that I'll cherish til the end of my days.
A skilled provider could be making it all up because she needs money for her car payments, just like a good somalier could be sick of wine and just putting in time on the job to support a family. But I think many providers tend to combine talent and a genuine enjoyment of their art.