On knowing extraterrestrials

Reading through one of the recent New Scientist issues last week I was amused by a line in a story about SETI and how the search for extraterrestrial intelligence must go on.

The story starts off with a comment that "[w]e live on a planet swamped by life forms yet we don't know how life got going, or where." Despite knowing so little of our own planet, the author later suggests that we know exactly the kind of signals that aliens will send to earth. He mentions the Allen Telescope Array that is "designed to pick up the kind of signal that an alien intelligence is most likely to send out into the universe: for example, a series of prime numbers transmitted in binary on a very narrow spectrum of radio frequencies."

I wonder, how exactly do we know what intelligent beings out there are going to be sending to us, if they do indeed exist, and want to send anything to their distant earthling neighbours?