There's a very nice interview in Astrobiology magazine with Pamela Conrad, an investigator at JPL, about searching for evidence of life via detection of indigenous complex organics. Dr. Conrad is much more of an Indiana Jane than I'll ever be-- she had to take rifle lessons against polar bear incursions during an Arctic expedition-- but the interview is very down-to-earth about her experiments.
The plan of attack is to look for a class of ringed organic molecules called porphyrins. This naturally occuring reactive molecule (seen on the moon, for example) is very good at electron capture, and is central to both chlorophyll and cytochromes (in mitochondria), which appear throughout earth life. It is also very easy to detect by uv excitation, which means you can mount a little robot with essentially a black light, and (on Earth at least) everything that glows will grow.
The reasoning in this and other searches goes that electron capture is so central to both photosynthesis and oxygen respiration that some analogous chemistry (also involving big, fluorescent rings) have got to be part of microbial life outside of earth. Patches of flyorescent regions can be inspected more closely, for example by looking for a bias in chirality of the molecules.
CORRECTION: In this post I originally said porphyrins function in electron capture. Actually a metal atom fits right into the middle, and that metal carries the electrons.