And one other question: Out of the whole radio spectrum, how do you know where to look?
Excellent question.
There are two real sources of noise that limits the radio astronomer's ability to search for very weak signals. 1) The Galactic noise halo interferes with us below 1GHz and noise due to earth's atmosphere interferes with us above 10GHz. This pretty much keeps all SETI searches (at least radio ones) between 1 and 10Ghz. Inside these two limits, the noise is around the 2.7K background from 1.4 to 7GHz. This is why most of the SETI searches are around the frequencies that the OH (hydroxyl) and H (hydrogen) molecules masers emit. This is the so-called water hole. OH H (tell me scientists don't have a sense of humor)