My helmet is plain white, so I put a bit of black tape on the back of it so the guy testing me could see when I was turning my head. Without that, it might not always be so obvious... So if you have a plain helmet, consider putting some kind of marking on the back to make it easier for the tester to see when you are turning your head.
Also, don't forget to shoulder check when pulling out, changing lanes, making a turn, etc etc....