look to see who trained the gym owner. Look for names like Fairtex, Vut Kamnark, Chai Sirsuite. See if they are Thai trained. If they have a ring, that is a major plus and an indicator that they are serious. Also, if they are offering BJJ there, they are probably serious as well. Alot of old kickboxers are trying to sell what they do as Thai because of the recent popularity boom.
try these links http://www.thaiboxing.com http://www.muaythai.com http://www.usmta.com
good luck