Maybe detecting a seasonal cycle for certain compounds will be a very strong evidence. Like the amount of atmospheric CO2 in air changes with seasons on earth.
That also happens on Mars as the ice (CO2) at the poles melts and re-forms with the seasons.
Methane similarly fluctuates on mars. Neither is strong evidence for life. But it remains possible, if unlikely, that some component of these fluctuations could have a microbial origin.