Its definitely really tough to do if the coach has no credentials AND no successful students. If you know the sport (the terminology, the moves, the rules, etc) but are just not very good or have no experience competing in it, then you can probably assess the coaches worth to some extent by observing. This is ok if the stakes are not very high. Meaning, if you want to get better at wrestling to improve your bjj, this is fine. If you want to win a state championship or better, probably not a good idea. Overall, youre gonna have to gather recommendations, reviews, test your results, etc.
There is also one other important factor. Are you improving under this coaches tutelage?