[maxxxxx]

There are plans for doing spectrography on the atmospheres of these planets. This would allow us to get a pretty good picture of the chemical composition. Biological life as we know it should leave a different footprint than a lifeless planet but in the end we can only extrapolate what we already know about life.

[civilian]

Yep! And more specifically, if we found a planet that had an atmosphere with O2 in it, that would be highly indicative of life. Although it's not the only option, O2 is pretty reactive, and you need a constant O2-producing force to maintain it. An O2-producing force would be something like plants or some life that is creating hydrocarbons from CO2 and H2O.