Like anything else in the capitalist world - look at the product.
Does the gym have fighters, or housewives doing cardio boxing?
If they’re fighters, are they fighting on the regular circuit of shows? Do they seem to have a good relationship with the coach? Are there guys that stay with the coach for long periods of time?
Does the coach get young kids who have NO IDEA what they’re doing, and turn them into fighters, or does he scoop up guys from other gyms that already know the game
You don’t want to go to a gym that doesn’t train fighters, and you don’t want a gym where the fighters are always getting their ass kicked.
That’s the good thing about boxing and muay Thai - the fakers get weeded out pretty quick because their fighters get their asses whooped. The good gyms, however, foster a “team” atmosphere and put out fighters of a basic quality - you can’t expect every kid to be a pro, but you can expect that the coach is going to teach them how to defend themselves, and not let them in the ring too early (unless he’s trying to teach them a lesson.)
Is the coach paying attention to his guys? Does he look attentive when they’re warming up? Do they look comfortable with him?
Visit a lot of gyms, and if possible, go to a couple shows that feature lots of different gyms. Watch which fighters look like they’re in good shape and have solid fundamentals… and you can usually tell solid fundamentals (at least in Olympic boxing) because that’s the guy that wins the fight.
Give that gym a call afterwards and see what they can do for you.
There’s going to be some limiting factors, particularly the price, the distance to your house, etc. but you should be able to find a solid one near you.