There are many documented cases of meteorites being reported as from Mars.
http://en.wikipedia.org/wiki/Allan_Hills_84001
While I can understand that it's possible to know if a rock originated as a meteorite. What I don't understand is how scientists can say it came from Mars with certainty.
How can they verify such a claim, and differentiate between a rock that is similar to one from Mars to one that really is from Mars.