I love Thai boxing. Some things to look for:
This goes for any sort of (Muay Thai, BJJ etc) a really good academy will have an alumni / 'fighter' section on website of all athletes who train there. I don't think you're trying to go professional or anything so this isn't really important ----
Most important thing will be transparency. Good places will let you drop in at least once and you can get feel for place. Things to look out for: is place clean, equipment (bags to use etc), are people actually sparring (very empty place or people just standing around is not good sign) / the frequency this occur, is staff forthcoming/answer questions especially about schedule & pricing (if just shove contract in your face without explaining much of anything is a bad sign). Are the trainers respected/legitimate? Doesn't have to be successful like Payakaroon but should be someone who loves the martial art and preferably has competed before (don't want like glorified fitness instructor who "dabbled" in Muay Thai one weekend).
Age of attendees ---> some martial arts places cater to more younger crowd (obviously you want adult oriented place).
Local reviews of place will provide you with enough information to make determination. Just a matter of then visiting and seeing if like the vibe. Use what above as things to look out for. Keep clinching