If we want to search Mars for life before we contaminate it then we need to launch a crash program to do so now, because our window is swiftly expiring. Right now a small group with minimal resources (on the to build and launch stuff into space scale) could likely launch a cubesat with some sort of bacterial payload that would eventually get to Mars. Such a stunt is only going to get easier in the next few decades.

Explore Mars, in depth, ASAP, or someone will render the issue moot by starting their own terraforming project on the cheap. (Or the not so cheap. I don't believe Musk's colonization plans -if they ever come to pass- would be able to prevent contamination, no matter how hard they might try.)