Muay Thai isn't the same as other Martial Arts. You don't really have the lineage thing to go on. Someone could train for years and have some fights and be very proficient and then open up their own gym. They may not be able to trace themselves back to an old Thai Master but this doesn't mean they aren't very skilled. The best thing you can do is train with him and see whether he's any good or not.
First time poster… I’ve always been a little dubious of the Ajarn thing. I’ve found the title of “Ajarn” or “Master” when applied to martial arts especially Muay Thai is typically political. They are rarely accomplished fighters but have been awarded this title through a “governing body”.
If you wanted to, I’m sure you could go to Thailand register for their course and eventually become certified as a Kru. Is that a bad thing? I don’t think so, if you’re not an accomplished fighter, you’re going to need some sort of credentials.
Do you need to be a former lumpini champ to be a good trainer? No, there are plenty of Kru’s out there who’ve got winning fighters, who aren’t fighters themselves.
Sorry for rambling but I completely agree with Zyph and Asriel. Don’t worry about the titles, and ranks.
Could be because of the fact that traditional muay thai (Wai Kru and Muay Boran things) are seperate from the fighting thing. That could explain why fighters don't hold or care about ranking and why the high level ones are not active fighters or fighters at all.
Muay Boran is almost extinct. The examples of it I've seen in Thailand were pre-arranged demonstrations.
Wai Kru is just a ritual and it practiced in some Muay Thai gyms still.