Both metabolic and super sensitive biosignature detection experiments to detect life would be confused easily by introduced Earth life or organics. This is just a taster of the many problems with searching for life on Mars. I'll talk about all this in detail later on, and about the many ingenious ways astrobiologists have devised to do this search in situ robotically.
Also, whatever life there is on Mars may be so sparse that if we go to the most habitable areas, perhaps we may find a total area of a few square meters of sparse, slowly metabolizing life, in an exploration region of several square kilometers. Or we might find a few spores in the dust, but perhaps have to examine a fair bit of dust to find them. So you are talking here about searching for something that humans may be unable to distinguish visibly, probably also in such sparse populations and hidden just beneath the surface, so that even with filtered vision it's impossible to detect it visually, and which requires very sensitive experiments, which would be easy to contaminate with Earth life and organics.