It’s a huge task. Data from radio telescopes largely consist of noise, a combination of cosmic static from the afterglow of the Big Bang, and emission from electrons whizzing around near black holes and other natural astrophysical phenomena, as well as noise from the telescopes and their electronics. In addition, there are many signals from intelligent life, some of them coming through so loud and clear that they essentially deafen the telescope, saturating the sensitive receivers at certain frequencies.
Unfortunately for astrobiologists, the intelligent life detected by Arecibo, at least as of June 2015, is not from a planet far, far, away. It’s actually right nearby. Be it planes coming in to land at the Arecibo airport, visitors to the site who have ignored the many signs requesting them to turn off their cellphones, or FM transmissions from Puerto Rico’s radio stations, all the artificial signals detected so far by Arecibo were generated by humans. But by searching mostly in a part of the radio spectrum that is preserved for astronomical use (at radio frequencies around 1.4 GHz), and by using sophisticated techniques to reject local interference, the telescope can scan the skies for faint signals from beyond our Solar System.