We went through this in the recent past with a meteorite from Mars. Electron microscope scans are difficult to hold up a definitive proof, because anything we would recognize as life would have to resemble forms of life here on earth. The naysayers would dispute a claim of proof, saying that, since it resembles a known form of life here, it must actually have been contaminated once it reached this planet.
The only conclusive way to prove the existence of life elsewhere is to find it somewhere else, like in the oceans on Europa.