heres a few things that can help you find a good class.
-the practicing of striking should be fast paced and hard. after you learn where to hit, a good school will train you on how to hit. learning how to hit can only be done when you are striking strong and fast. this can be done in the air or against targets. avoid a school where everything they do is slow (i call this demonstration speed) especially the countering practice.
-during sparring, does the teacher stop them often to give pointers. or do you see him observing one fight at a time to give advice to each guy. lazy teachers put the whole class in sparring together, and he doesn't really coach anyone in their fighting styles. so really they are just killing time.
-a good class can be a learning class, where the teacher is explaining things and gives each person a chance to figuere techniques out on their own (slow pace, lots of explaining). or it can be a training and developing class (fast pace, lots of repetition, and little explaining). or a self development class (everyone at his own pace, with a few breaks to show something new). the best way is each of those classes are basic (the slow one to learn a new technique), intermediate (the fast one after they learn it), and advance (once the skill is developed, they make it into their own style). the best classes have just a few things covered in each class. the worst ones try to cover everything, if your goal is to be a developed fighter.
now this is only my opinion of a good school. dont take it to strong, because understand that as a beginner, you cannot really judge if a teacher is qualified or not or if he cant teach you well. its like a guy who doesnt speak spanish, but he wants to see if his spanish teacher is any good. you wont know until you now more than him or just as much. i seen teachers who were very overweight, had a boring personality, but these guys were excellent teachers. i know a lot of guys who are in very good shape, they look good when they train, even they are good fighters and they suck as teachers. but you can learn from anyone. the point is you find a teacher you like to learn from, and you suck up all the learning he can give you. if the style you end up with is not what you wanted you can allways learn another one, but you have to give it time to develop inside of you.
my last advice, dont school-hop. guys who dont have the discipline to stick with anything for long...no matter what the excuse is (style is not for me, dont like the teacher, looking for more "realistic" combat, whatever) those people never excel in anything. stay with one long enough to be an expert in that. at least you will have the discipline to excel in the next place if you move on.