If live bacteria were found on Mars, would there really be a question over whether they were mutated colonist bacteria or native Martian bacteria? Probably not; the difference between a few years of mutations and a few billion years should be obvious. Still, it's possible that there would be some convergent evolution in Martian bacteria, and some genes similar to those on Earth might have evolved.

Of course, this would only be possible if Martian life was basically the same as Earth life, or if both sets of life it had the same original source. This would be possible (indeed, some scientists think it would be likely) because of material exchange between the two planets.