If the alarm goes off, the hostie stops and looks for the likely suspect or asks all the people in the nearest seats who has an electronic device and tells whoever it is to turn the device off. The hostie doesn't have to decide if some smartarse who wants to listen on a scanner is trying to convince her that it only an MP3 player or whether device A is OK and device B isn't OK. If her instruction is that the device is to be turned off, then it's turned off!