A simple little thing for a coach to do during drills or scrimmages is for the coach to hold up a number of fingers - 1,2,3,4,5 - then have your players freeze and pull down your hand. See how many of your players know the 'magic number'. I usually reward the ones that do know with a treat. During scrimmage, I only see if the ones that are immediately around me know, since it's too easy for the far away players to stare at my hand! Simple but effective