Meteorites from mars are exceptionally rare. Gravitational interactions amongst all bodies in the solar system cause shifts in trajectories and some of these shifts may cause large masses to collide into one another. When one of these giant collisions take place on Mars, the low atmospheric pressure (1/10th Earth's) present allows many of these objects to speed up to escape velocity and be hurled into space.
Once considered improbable, theorists were forced to reconsider their stance on this phenomena because of comparable findings between specimens on Earth and findings from the NASA Viking spacecraft which landed on Mars in 1976. Instruments on the spacecraft measured amounts of different gases in the thin Martian atmosphere which were then found in very small amounts trapped within shocked glass veins in the shergottite Elephant Morain 79001 by Donald Bogard and Pratt Johnson. These have now been seen in 5 other meteorites.
Other meteorites that are suspected to be from the Red Planet but do not contain this atmospheric gas are measured against other diagnostic criteria. The most significant of these tests is one that reads the range of oxygen isotopic compositions which is much different than any other achondritic meteorites. As of the time of writing this, there have been as many as 142 meteorites believed to have originated on Mars.