While we probably can't rule it out altogeter, I think it would be possible to show that Mars quite likely did not have life. For example when we have looked at carbon and sulphur deposits from throughout Martian history and found none of the isotopic signatures of biological processes, then we will be able to say that liife is unlikely to have occurred. But we would have to have such data from many locations and spread over much of Martian history before we could do this.

Jon