It was recently reported that the Martain atmosphere contains methane. Because methane is unstable in the atmosphere, the implication is that life on Mars is generating methane. No flatulent cows are in evidence, so presumably these must be methanogen soil microbes.
Methanogens generate methane
by reducing CO2 - this produces O2, which must either be excreted with the methane or bound up in the organism. I propose that areas of Mars thought to be promising for life be doped with CO2 containing radioactive oxygen. We could send up a few extra cruise missles containing big tanks of pressurized labeled CO2. If the CO2 is being consumed by soil bugs, they should generate radioactive O2.
This radioactive O2 could be detected by a probe sent up a few years after the CO2 lands. An excess of radioactive O2 in the area as compared to other areas would suggest methanogen metabolism going on there. It may even be possible to use a device like a geiger counter to find pockets of radioactivity and sample them - presumably bugs that have incorporated the radiolabeled O2 into their tissues.
I am a little concerned that the radioactive CO2 might just blow away. These missles might need to embed themselves into the soil then slowly leak the CO2 into the subsurface.
Mind you - this is low level radioactivity we are talking about. No bugs will be hurt by radioactivity.