How Do You Distinguish Between People-Made Signals and Those From Space?

Question: You have loads of terrestrial interference, so how do you know the signals you receive are extra terrestrial? — Ariane

Answer: The radio signals that we generate ourselves, such as by radar or radio communications, have a specific frequency and frequency width that we can measure very accurately.  Signals which come from astronomical objects are much weaker, come from a specific direction in the sky, and often have a spectral shape which is very different from those that we measure from astronomical objects.

Jeff Mangum