There are a lot of Muay Thai gyms around where I live, so I had a lot to choose from and I checked out a few before settling down in the one I'm training in right now.
For me, the first factor I based my opinion on was if it is on my way to work, or near where I live, so I can go there with no excuses for rain, cold, snow, sun... ;)
The second factor was, if there were women-friendly classes.
The second one was only important at choosing my first gym, though. At that time I wrote to them via facebook, spoke at the phone and decided to give it a try after learning that half of the group are beginner girls!
And this gym was a really good match for me! Back then, they were just starting out the gym as no names, but I liked the way the practice went on. Right now it's one of the most popular and growing gyms in the area :D
I quit my job, so I had to choose another gym, and this time I did choose based solely on the location. I went and checked out 2-3 other gyms, but still, the deciding factor was the location :)
If you're a beginner in martial arts I'd advise you to go to a gym near you, feel out the atmosphere and take part in a few training sessions. The first sign that everything is great will be if your physical fitness starts to improve a little bit by every session, you start to remember the technique and after a few weeks into training - if you are being taken care of. No trainer wants to invest his time into beginners that come 1-2 times and quit. If you prove that you want to seriously do this, they will start to teach you properly.
I also noticed, that it's better to go join a beginner group that stated it's a beginner group :) At my gym, we have an "open" group, which means anyone can join at anytime and there are beginners and veterans in one room training together. It's not the best solution, but it's ok for me, for now. I'd strongly advise a beginner to go to another gym though.
I don't care much for cheesy advertisment. I might try out a gym if I think I will like it, because it's in training where you learn if the trainer is a cheesy douchebag or not ;)