There are already systems proposed to rank the likely habitability of exoplanets, but these are aimed at finding places where we'd like to live, rather than whether life exists already. In Astrobiology Bains and Seager make a general argument that Earth's living systems use redox chemistry not only to capture energy but to build biomass; and that furthermore, in contrast to energy capture, biomass processes must convert fully oxidized or reduced compounds into a more intermediate range. This may be a general feature of all living systems that use carbon. Because the effect of biomass is the most prominent directly observable feature of life on Earth (the absorbance of forests, for example), looking for large amounts of intermediate-redox compounds might give us a general way to detect carbon-based life elsewhere in the universe.
My own $0.02: since biomass is not likely to accumulate until life has evolved some complexity, it may help to narrow our search to ecosystems that have progressed beyond the single-cell stage. But it would be good to find one other ecosystem, of any kind, first.