If an astronomer picks up a radio signal it is extremely obvious whether it is an artificial source or not. Natural sources have very inefficient spectra of very specific types. Artificial sources have very clear frequency cutoffs, having come from an antenna. Even frequency-hopping sources may jump around in a way that makes eavesdropping difficult, but if you stick around long enough on a single channel (as radio astronomers do) you'll get little blips now and then as the signal hops through your channel and is gone again.

How do we know this? Because astronomers see these every single night and have developed sophisticated filters to remove them. But by checking how the source is moving it is quite easy to tell if source is local or not, and sure enough all these artificial sources come from satellites, the troposphere, or an unshielded microwave.

It is actually remarkably difficult and a challenging problem in itself to create a communication mechanism which looks like a natural source. It defies imagination to think that every single intelligent species out there has purposefully masked their existence in such a way, including giving up useful technology like ground control radar or mobile communications.