Here is an excellent overview for a general public audience of the question, “How to Find an Inhabited Exoplanet”:

The speaker is David Charbonneau of Harvard University. He gave the presentation at the Space Telescope Science Institute in Baltimore, Maryland for one of their public lectures:

Over the past couple decades, research has uncovered thousands of exoplanets in orbit around other stars. In particular, the NASA Kepler Mission taught us that Earth-sized planets are commonplace throughout the Galaxy.  But the big question remains – did life take root on any of these distant worlds? Using upcoming large telescopes, astronomers will examine the atmospheres of Earth-like planets for the telltale chemical fingerprints of life. The search for life in the universe has entered an exciting new phase.