system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know we aren't seeding life on Mars when sending probes there?
gpt
Simple answer, we don't, we actually assume at this point in time that this is exactly what we have done. 

NASA has already stated numerous times that some of the equipment that was sent was not completely sterile, due to mistakes and errors and that they believe that they could have contaminated the soil.

They believe that the life that they found, was in fact from earth.