They don't have to survive for you to find traces.
> Results of the scope of scientific experiments which had been conducted for a quite long time were summed up in the previous year, confirming that some organisms can live on the surface of the International Space Station (ISS) for years amid factors of a space flight, such as zero gravity, temperature conditions and hard cosmic radiation. Several surveys proved that these organisms can even develop.
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.