I was on a flight once on which the captain announced that we couldn't push back from the gate because someone was using his cell phone. I could never figure out how the pilot knew that, unless the FA spotted someone and reported to the captain rather than just telling the passenger to shut it off. I also suspected that perhaps the captain had no idea whether someone was using their cell phone but rather made the announcement anyhow just to scare people into turning their phones off (the equivalent of dad shouting, "Don't make me come back there.").