They'd have to know the specific receiving frequency which could be determined with an antenna hooked up to a frequency counter. Every time she received a transmission the frequency would be detected. What would be "more" fun though would be use a ham radio tuned to the same frequency and let everyone hear the signal she's receiving during the broadcast without her being aware she's been caught...lol
Twenty years ago there was a receiver made by Optoelectronics available to the public that recorded frequencies of transmitters in the area. It was the size of a pack of cigarettes. The frequencies came up on a display.