Easiest metric for whether a gym is legit or not: do active, competitive boxers train there?
Also depends on what your goals are; if you want to be competitive, or if not actively competing, being in that kind of shape.
If you just want some time on the heavy bag and focus mitts, and some circuit routines thrown in, any old gym will do.
If you want to be competitive, or be on-par with people who are, gotta go where they train.