A good way to start is to look at certifications. There are many organizations out there, but the general attitude is that the National Strength and Conditioning Association (NSCA) and the American College of Sports Medicine (ACSM) are the only worthwhile ones, since they focus on a much broader spectrum of topics.
The important thing here, though, is that understanding that certifications do not equal actual knowledge. Think of it like a driver’s license. A 16 year old kid that just got his license is just as qualified as a Nascar driver. Does that mean they’re just as smart? Hell no. Same thing. By only looking at a NSCA-CPT or a CSCS to train you, you’ll be able to weed out a lot of losers.
Like others have said, they should be able to explain what they are always doing to expand my knowledge. Continual gain of knowledge will really show a person’s dedication to their profession.
Also, ask them who they refer for psychiatric needs or physical therapy. If they don’t have anyone, they don’t have a very good understanding of their profession. Personal trainers are not trained to handle psychiatric issues (eating disorders, which are common among females in club sports) or to rehab injuries.
Here’s my personal thing: Ask them who their trainer is. No matter how good of a trainer you are, there’s just some stuff you can’t see when you’re lifting, and having another opinion from another expert is invaluable. Doctors have doctors, and lawyers have lawyers. Why shouldn’t trainers have trainers?
Lastly, I wouldn’t discredit anyone that doesn’t look like a fitness god. Some people just don’t have the time, energy, dedication, or desire to be super ultra built. Obviously your trainer shouldn’t have a beer gut and perpetual Cheetoes-stained fingers, but it’s not like you have to be 260 pounds at 5%bf to know how to train people.