I'm not the best person to give advise about groomers, I tried one and then gave up and have been trying to cut her on my own.
But like the previous poster said, get the basic info (their policies, their cost, check out the environment, etc) and get a vibe about them. I can honestly say I visited at least 3 groomers that after visiting and asking questions I already knew I'd never take Zoe to them. Also, ask around, if you have any friends as who they use!
Good luck!