Unlike the meteorites which have been determined to have come from the Moon. Those from Mars have not had that determination made by comparing them to other actual samples. No sample return missions have yet been seen to Mars. So how do we know that Martian meteorites are truly from the Red Planet.
While we have not sent back any samples of rock from Mars we have sampled its thin atmosphere and know precisely what gases are there. Rocks often contain gases that are trapped in them since their formation. This is the case with Martian meteorites. By crushing the meteorites and analyzing the gases they have been holding for ages we have found that the unique ratios of isotopes of several gases are the same as those found in the Martian atmosphere. They are ratios that are very different from the ratios of those gas isotopes found on Earth. Since the rocks are types produced by geologic processes found on planetary size rocky bodies Mars is the best candidate and the ratios match as well.
Perhaps we will soon have a sample return mission to Mars and we can learn even more from those samples. But, until then scientist are making some great discoveries working the significant number of Martian meteorites that have been found right here on Earth.