Setup a radio in the room playing loudly, then go outside the room with a police scanner and scan through all frequencies to see if you hear the radio broadcast. Unless they were made and placed by a true professional, they're going to operate on common bands (900Mhz, 2.4Ghz, etc.). Unenctrypted and unsecured covert devices can be easy-ish to find.
The technology has changed, but some of the basic concepts haven't.
I think it was first published in the early 80's, my copy is from '89 or so. If you want a fun read, look for 'How To Get Anything on Anybody' by Lee Lapin. This used to be a little easier, back in the heyday of some of the first covert electronic surveillance devices in the non-government market. This is one of those scenarios where a positive hit (a found device) certainly indicates a true listening device, but a negative hit (no results) simply means you didn't *find* anything, not that no devices are present.
