Most of this advise is a sub-set of looking out for a Mc Dojop. I am going to assume that you are referring to technical knowledge instead of generic "should I train here" which has been answered here and here.
- Are a majority students showing signs of injuries? If so, run.
- Is the teacher explaining techniques safely?
- Is the teacher explaining techniques clearly?
- Is the teacher humble or arrogant?
- Are students practising with you appropriately for your level?
- Is there a warm up and cool down in each class?
- Is there a fist aid kit in the gym?
- Is there water in the gym?
- After the class: do you hurt more than expected after exercising hard?
You could look at how many fighters went on winning tournaments within your gym? This can give you a clue as to how well people coming to train do. Although, this might be a false alarm is the gym is a fun place without pushing fighters. The class might still be really good. However, I would expect anyone teaching Muay Thai to have had a career as fighter (maybe just as an amateur) which you could check…
Note that you cannot tell if the technical knowledge is "correct" since you have no idea what Muay Thai correct is. However, use your common sense to determine if the techniques you were shown are practical. As a hint: the more complex the technique, the less practical it is.