How do we know Martian meteorites are from Mars?

Hi there all, love the suite of shows!

There are a number of meteorites which astronomers classify as Martian. First of all, how do we know they come from Mars?

Secondly, a few months ago, there was an article on the Naked Astronomers about a discovery on Mars itself in which it was said that the sample on Mars was an anomaly as it did not match the Martian Meteorites. It struck me as odd that the meteorites were being used as a reference rather than the actual geology ( or should that be marology) of Mars. Can you explain?

Wishing you every blessing
Fr Gerry Drummond