There's a few ways to detect radio receivers, some of them based on listening for incidentally radiated intermediate frequencies (which have to be a multiple of the received frequency) from the amplifier in the receiver: https://en.wikipedia.org/wiki/TV_detector_van

It would be a heck of a trick to detect a GPS receiver that way, though. Probably be easier just to listen for the RF noise from the switching regulator in the power supply.