Labeled release experiment
"The LR experiment moistened a 0.5-cc sample of soil with 1 cc of a nutrient consisting of distilled water and organic compounds. The organic compounds had been labeled with radioactive carbon-14. After moistening, the sample would be allowed to incubate for at least 10 days, and any microorganisms would hopefully consume the nutrient and give off gases containing the carbon-14, which would then be detected. (Terrestrial organisms would give off CO2, carbon monoxide (CO), or methane (CH4).)"
http://cmex-www.arc.nasa.gov/MarsEssy/life/life.htm
Result: Labeled gas emitted.