
Better to go by word of mouth a lot of the time. There are many relatively recently qualified groomers round here, and several do a really bad job. I saw a young westie last week who'd just had her first groom, and she looked absolutely appalling. Home job with clippers from Argos appalling! I'd never have thought she'd been to a groomer.
Also, I take care with how I groom dogs and I get great feedback, and I learned the official breed styles properly, but I have no official qualification. There's a certificate somewhere to say I completed a course, but it's not accredited or anything. Word of mouth is the best way, really, especially if you come across someone with your breed that looks like it's been to a good groomer.