Most media buyers do in fact verify their media buys. There's an entire cottage industry of people that actually watch tv programs to make sure commercials ran, drive out to billboards to verify the right ones were put up, etc. While I haven't bought a ton of podcast inventory I have to imagine that this is done for that medium as well.

I happened to run into a movie trailer verifier in the lobby of a theater the other day. It's something I hadn't really considered before, but really of course someone will be verifying movie trailers.