The origin of the meteorite in which NASA scientists identified micro-fossils, seems to be Mars on the basis of an isotopic study of trace gases locked up inside the meteorite. The isotopic abundances match exactly the martian ones which were recorded by the Viking lander, and like fingerprints, it is believed to be practically impossible to create the same isotopic abundances elsewhere in the solar system.
As for being 100 percent, absolutely, positively certain, well, that's not what science is all about. All we can ever say for one-shot phenomena like this is that the data currently point in that direction. The evidence for a martian origin seems conclusive, even though we do not have eye-witness video recordings of the actual event 13 million years ago to PROVE this is where it came from.