I'm not sure how they know it but my last flight on Southwest out of Orlando the kid across from us had his game on right before we took off and then the capt came on saying they "detected a prohibited electronic device in operation" he asked to have it turned off or we were not flying. the kids parents turned 5 shades of red took the game away and then the capt came back on and said "thank you, prepare for take off"

Now how did they know it was on from up front?