What do we do if we find something??
How would you know? The main feature distinguishing signals produced by a transmitter from those produced by natural processes is their spectral width, i.e. how much room on the radio dial do they take up? Any signal less than about 300 Hz wide must be, as far as we know, artificially produced. Such narrow-band signals are what all SETI experiments look for. Other tell-tale characteristics include a signal that is completely polarized or the existence of coded information on the signal.
What if we detect something?
Confirm that it's truly extraterrestrial! An important test to
verify that a signal is truly extraterrestrial would be a
confirming observation at another radio telescope.
Once an artificial signal is confirmed as being of
extraterrestrial intelligent origin, the discovery will be
announced as quickly and as widely as possible.
The Protocol
A
Declaration of Principles Concerning Activities Following
the Detection of Extraterrestrial Intelligence, endorsed by
six international space organizations, describes how to make such an
announcement. There will be no secrecy, and indeed getting the
word out quickly is important, as there would be an urgent need
to have astronomers world-wide monitor any detected signal,
24 hours a day.
In order to understand anything that E.T. might be saying to us, we'll have to build far larger instruments to look for the modulation, known more colloquially as the "message", of the signal. It's more than likely that, once a detection is made, the money will become available to build this far larger instrument.
Until we can measure the modulation, all we'll know is that there is intelligence out there. We can pinpoint the spot on the sky where the signal is coming from, and slow changes in its frequency will tell us something about the rotation and orbital motion of E.T.'s home planet. Even with this limited information, the detection of alien intelligence will be an enormously big story. We'll know we're not alone, and we're not the smartest things in the universe. And of course there will be a loud clamor to build the big dishes that would allow eavesdropping on E.T.'s message.