Firstly I would like to start the question with the knowledge that we have, that the most basic form of life we want and expect to find in our Solar System is the primitive kind, such as viruses or bacteria, since they have the highest chance of existing, more than intelligent life (Fermi Paradox).
Now with that in mind we also know that viruses are also half alive, and half dead, and they are really tiny, much tinier than bacteria or cells. These viruses are everywhere around us, even in the air, and also, in our capsules to where we send probes to Mars.
Since our last probes that got sent to Mars, many years have past, and many more will pass until we really check for microscopic life on Mars, meaning that these viruses might mutate and evolve, maybe even bacteria could do that (I am not a biologist, please don't quote me on my ignorance). How would we know and differentiate between life brought by us, or real life in the red Planet?