Most, if not all PTs that belong to commercial clubs are worthless. To determine if a PT is good or not, there are several criteria to consider
1) His background - was he a previous athlete? If so what? How many years of training experience? How many years of coaching experience? What's his specialty/niche? Fat loss? Hypertrophy? Sports performance? Training women?
2) His portfolio - previous client testimonials and results(before/after pictures, etc)
3) His training style - does he just go through the machines and count reps for you and/or stand by your side while he watches you run on the treadmill? If so, fire him. A good PT will teach you free weight compound exercises with barbells and dumbbells. Any PT that doesn't do this, and says that they are somehow dangerous or will make you "bulky" deserve to be fired.
4) Physical assessment - this should take up 1-2 sessions by itself, and will access your current physical level, including strength, flexibility, mobility, muscular balance, postural distortions, address existing pains, etc. With these information a PT will then be able to formulate an initial workout plan for you. After this, physical assessments should be taken at fixed intervals(e.g. every 6 weeks) to measure progress.
5) Nutrition - Obviously a big factor especially if you're training for physique. He will probably require you to keep a food log for a while, and educate you on healthy nutritional choices.
6) Goal setting - without a goal you're going nowhere, so a good PT will assess your goals, and together with the physical assessment, put together a training program that works for you. Specific goals may be to go from 20% body fat to 10% body fat in 6 months, or to increase all major lifts by 20% in 3 months, stuff like this. There needs to be a timeline for accountability.
7) Coaching skills - how good of a teacher is he/she? Since the bulk of your interaction will be him/her teaching you how to perform the lifts, he/she needs to be good at it. This means having intimate knowledge of the various lifts, as well as a good eye in spotting form problems and how to correct them. This only comes with experience.
Motivation - how motivational is he? There are some clients who just simply can't be motivated, but most people who are willing to spend the money for a good PT(good PTs usually price themselves beyond the price range of lazy, unmotivated people) will probably want it bad enough such that some motivation will work.
9) Money back guarantee - this one's a big one, because it shows the PT's confidence in his/her skills and knowledge. Not a lot of people do this, but the ones that do are usually great.