There was a experiment run on the old lander that had a positive result for some amino acid.
That is all I recall.
Better proof would indeed be a fossil, or microscopic evidence that is un-refutable.
I do not know for sure, if they will get anything like that this outing, but we continue to speculate.
Now, the water has always been talked about, but not located. If water is sub surface, then fuel can be manufactured from it.
That is what is important for a manned flight.
Yes I believe Bill Clinton told Dick Morris about life on Mars (before the rest of us knew) as Morris having his toes sucked by a high-priced hooker.