Microscopy could also be a good route. Microbial life should look pretty distinct from inorganic matter, even if everything in the soil sample were dead by the time it was imaged. As far as I know, we have not sent a microscope up capable of resolving even a large mammalian cell, let alone resolve 1 micron bacteria (if these microbes are even to be similar in size as those on Earth). I suspect the vibrations during launch make it especially challenging to actually get a powerful microscope out to Mars unharmed.