Your sub conscious is actually pretty good at doing a lot of the work for you. Therefore visit the schools, spend some time there chatting to people and just getting a "general feel". Your senses will tune in to your new surroundings and you will build an opinion on the place. Don't rush it though.
Repeat this at quite a few schools and the combination of all your research will make your decision making a lot easier.
I know it all sounds a bit "hippy" but I found it worked. I walked into one school and immediately knew it was where I wanted to train. I of course stuck to my system of chatting to instructors, students and asking the important questions.
Oh and NEVER pay up front. I would say a small deposit is acceptable but if anyone is asking for more than that then thank them and walk away. It's tough out there and I'm sure there are a lot of places out there who above water look calm and relaxed but underwater are paddling like crazy to stay afloat.