Steve, cheeks should not move. Those old self taught players with the bugling cheeks ruin their neck muscles. Nothing deadly, but not really a good thing either. You shouldn't be able to tell anything from the cheeks. Maybe from the way the stomach moves. (but even them movement is slow). Get close though and you should be able to tell if he is playing or not by when he breaths in. Wow, I only had trumpet for one year in 5th grade, I'm surprized I remember that much.