free trials/money back policies are usually a good sign. If you can find any other martial arts that allow free trials go to them all and see what one's fit and what doesn't. Every class is going to have people that seem like they excel to someone without training but the best way to figure out who's worth training with is to see how they compare to others. If you're in a good class no one is going to be offended if you tell them you're going to be trying other classes or if you ask for credentials, if they stake a huge claim you should be able to google their name and find something. You'll see the red flags by being in the class better then anyone on the internet.