If there are signs of life on Mars it's unlikely that we'll be able to see it with any imager on the rover, but we might be able to detect its chemical signature. The sample analysis instrument is a gas chromatograph connected to a laser spectrometer and a mass spectrometer. What happens is that a sample will be vaporized and then pushed through a long column. The column is designed so that different molecules will have a different speed through the column and thus will come out at different times. Then each of those parts of the sample (which should be comprised primarily of one molecule) is passed into the spectrometers, which can identify its molecular structure. This can determine the chemical makeup of minerals, but it can also identify complex organic molecules such as the components of cells or perhaps things like cellulose or lignin and so forth.