The fastest way isn’t to eliminate the other possibilities, it’s to find a unique test for a that can’t be met by any of the others. Bear in mind that your list of alternative causes may be very long and each require large amounts of work to eliminate.
I too find it strange that multiple landers have been sent to mars since Viking, ostensibly looking for evidence of present or past life, and have yet have carried no tests for it. I find the best way to find something is to look for it.