Panspermia is a theory for a reason. As an explanation for life on Earth itself it has a challenge ahead of it, but due to the small amounts of life that Earth is known to be flinging about it's a known fact that Earth life has landed on the other planets. (Or at least their corpses, but it seems reasonably likely that some of it would survive the trip in some form of stasis or spore, given how resilient we know microscopic life can be.)

If life is ever found on Mars, the next question to be asked after identification is, "is it related to Earth life?" Which we'd check by checking certain of the parameters of life on Earth that do not seem to be "forced" by physics, but are universal on Earth (how DNA codes for proteins, for instance). If they all matched up, it would strongly suggest one single common origin... though that origin could conceivably be on Mars.

Interestingly, this also means that if the Solar System is still otherwise sterile despite the arrival of Earth life, that has interesting implications of its own.