Mars is the only planet providing rocks with a similar chemical composition and age. So the origin of the meteorite is evident.
This doesn't mean, that it's absolutely waterproof. There might have existed protoplanets similar to Mars 4 billion years ago, which since then have been swallowed by Jupiter, the Sun, or have been ejected out of the solar system. But this would be much of coincidence without leaving convincing evidence. (Here a paper about a couple of simulations.)
In addition to the chemical composition and age of these meteorites mentioned in @Gerald's answer, there is another clue in martian meteorites that makes it very unlikely that these meteorites might come from another body: as the rock gets partially melted, re-solidified and ejected into space, small bubbles of gas get trapped in it. These gases trapped in the meteorite have been analyzed and compared to the composition of the martian atmosphere measured by the Viking lander, and they are strikingly similar.