Different criteria applies for different people. Depends what do you want to achive.
In my first year of thai boxing I didn't really care about the techniques all I wanted was to lose the extra weight so I went to the closest muay thai gym, learnt the basic moves and then all I did was rope skipping, sandbag/pads punching and kicking (basically a lot of cardio) which was quite fun because the students were not at the same level.
After that I got more interested into the sport and found a better school, where there are real thai instructors, they have a programme and most of the advanced students are amateur fighters so I learn a lot from them.
If you have time visit the gyms and ask about their instructors/training/facilities/fees and compare.
If they train fighters for the 31st of March event they must be good
On the other hand don't be fooled by the aerobics-kicks-punch-burn classes - usually they have madonna or britney in the background
Hope this helps.
Oh my, sry for the long post