Personally I looked a lot into offsite reviews/what information they offered online, then if I was interested phoned in to see what else they could tell me or if I could get a consultation. The place I chose offered discount coupons for good Google reviews, so I looked at the reasons for any lower reviews/read the more detailed ones instead. They have in-house doctors and the tech's are all medical professionals in training so I felt a lot better with that opposed to the average beauty salon. It's really nice if you can find a place that offers per-appointment payments instead of locked in package deals in case you end up disliking them, too.
A couple things you might want to ask:
Do they have up to date equipment
Do they lower/heighten the treatment strength based on results/client comfort
Do they offer a numbing gel (this might cost extra, you can buy from your pharmacy usually if not) before treatment?
W/e else you're curious/concerned about
They should also check/ask your medical history before each session (make sure there's nothing conflicting going on - Ie. You haven't been on Accutane in the past year)
But honestly it's not as sketchy or invasive as you'd think, assuming they practice proper hygiene and such. It does hurt like a bitch on thick hair but worth it.