There are postures in Yang Style that are traditionally done wrong by schools that have no martial aspect.
Brush knee might be easiest to tell. If the hand ends laterally (left or right) even with the shoulder then it is exercise. If it is laterally even with center of chest then it might be a martial school. However, this "test" may not be 100% valid, since on bad days good people still mess up moves, and in teaching beginners he may not focus on this when doing the move.
Even if it is only for exercise, you can read the classic books, hit the internet, magazines, etc. and still learn a lot. If what you read from real experts out of class match up with class, then good. If it is obviously different (such as TTCS) then you are only doing exercise, which isn't entirely bad either. I started with an exercise only Tai Chi, moved on to a more martial and most of it transferred, but I would have saved time starting at the right place to start.
Brush knee might be easiest to tell. If the hand ends laterally (left or right) even with the shoulder then it is exercise. If it is laterally even with center of chest then it might be a martial school. However, this "test" may not be 100% valid, since on bad days good people still mess up moves, and in teaching beginners he may not focus on this when doing the move.
Even if it is only for exercise, you can read the classic books, hit the internet, magazines, etc. and still learn a lot. If what you read from real experts out of class match up with class, then good. If it is obviously different (such as TTCS) then you are only doing exercise, which isn't entirely bad either. I started with an exercise only Tai Chi, moved on to a more martial and most of it transferred, but I would have saved time starting at the right place to start.