A shop or salon that displays membership with an organization like this is always a good sign.
National Dog Groomers Association for Professional Pet Groomers
The requirements for membership are different for each of the groups but if they (shops & salons) care enough to belong to one of them that's a good sign.
Also, some of the best most compassionate groomers around never went to a "school" but apprenticed with an expirienced teacher.