Some people still think that all personal trainers are 'roided up' big heads, when that isn't true. Sure, there are many trainers out there that spend more time flexing in the mirror than concentrating on their clients, but not all are like that. So, to start with, you want to check out someones approach to you. If they seem cocky, arrogant or don't seem that interested in your needs, then walk away, fast! Some trainers go on an on about how they have transformed peoples lives, what they've done, their achievements, blah blah blah. That's all well and good, you want to know the background of a trainer, but if all they do is talk about themselves, then find someone else, personal training is about YOU, not them. You have the right to ask them whatever questions you want - their background, how long they have been a PT, their training etc etc, but if they love to talk about themselves all the time, its a big no no.
Another tip is to ask around to see if anyone recommends a PT. Referrals are crucial to a good trainer. If nobody seems to recommend a trainer, then that says it all really doesn't it.... If a PT is good, people will tell people! they will tweet, Facebook updates, text, etc etc, so ask around! A thing to remember is, if a PT is busy, then that usually means they're good. It shows that they are having regular clients, and regularly getting new clients.
It's actually quite common for people to change trainers early on, and that's because after a few sessions you will get to know what the trainer is like. So if your trainer isn't punctual, polite, 100% interested in your needs, concentrates on your session 100%, has his phone out (unless there is an emergency of course...) then it's time to change trainer. The whole concept of 'personal training' is that it's personal. If you have a trainer that plonks you on a treadmill and disappears for 15 minutes, heck....I wouldn't even finish the session! I would walk out then and there! You want to feel like you are getting value for your money, after all, personal training is cheap.
You want to hire a trainer that is presentable. Now, I'm not saying that he/she has to spend a fortune on clothes and equipment, but if they look scruffy, and seem like they just chucked on any old t-shirt and trousers, then look else where straight away! If a trainer can't be bothered to 'look the part', then I doubt that they are any good. There maybe a rare occasion where you have a fantastic trainer that looks a mess, but as I don't personally know every trainer in the world, I'm speaking in general terms.
The most important thing to look for in a trainer, is that he/she gets results! Whether it's shown on their website, Facebook page, blog, posters, or whatever else, make sure they get results! The trainer could look the part, talk the part, act like the worlds best trainer, but if they don't get results, then it means they are NO GOOD! It's like someone who plays golf - they can go and buy every single bit of clothing and equipment that Tiger Woods has, does that mean they will be amazing? hell no! usually means they're an idiot....
Be smart, ask questions, and remember - it's your money your handing over, don't waste it on a wannabe.