It’s a dead giveaway when nearby students look at another student quietly occupied and then look at the teacher, so I walked quickly but stealthily back the center aisle, and now the offending cell phone user, who should have been reading his novel, has a three-day vacation from school.
Another giveaway is leaning one’s torso back or to one side, or maybe twisting one’s back uncomfortably, all the while looking down into one’s lap to tweet or text or do whatever; it works about as well as the proverbial ostrich’s hiding from the world by ducking his head into the sand and raring his rear.
It looks sort of like this…presidential example: