I am absolutely certain we will be able to distinguish earth based life forms from those that have spent any relevant amount of time off of earth. We can shotgun sequence seawater and distinguish organisms we've never even seen before. I'm also certain we are not successfully decontaminating our spacecraft. Why do we waste the time and money failing to decontaminate what will essentially be our first terraforming efforts? We desperately need our microbes to travel in advance of us. Let's go!
p.s. Yes I know one of the main talking points from the green mars series was to slow down and do it right. Some people might want to leave it pristine so we can do more research. meh, learn by doing. Earth microbes for life!