I have become expert in the weird little skill of detecting phone use in class. There is a certain hunched, downward-looking pose that is a dead giveaway. I am also fly to the move in which you tuck you phone under one leg when I am too close kids!
I have a rule against smartphones in class. The rule is not because I am fundamentally against them, is is because I am much more comfortable being asked for permission for specific uses: the boy that has an easier time concentrating if he can listen to music, or the girl that uses an online book of answers AFTER she did the problems. I am fine with those uses, if asked politely, but gaming or surfing on the phone will cause it to become mine and I am not always nice enough to give it back at the end of the period.
The skill has gradually become acute enough to yield some false positives: one boy had already lost his phone to me and was playing with the cord of his hoody (he said it and I did not care to challenge it, they are all teenagers) and one girl had a hand full of small change.
She did not want to hand the money over, which was a shame because of course teacher pay being what it is....(taboum-tzing).