We cannot be absolute, but when we analyze probability, namely how many Solar Systems there are, it seems incredibly likely there is another biosphere. There's another biosphere in our solar system, one of the moon's of Jupiter (with ammonia as a viable liquid medium). However, just because there's probably billions of biosphere doesn't mean life started given how small of a chance that is to happen (I forget what the percentage is). Still, given the fact there is so many biospheres, and the probability of life is based upon the chemical composition of earth in which another planet may have a biosphere more optimized for abiogenesis, it is quite possible there is another lifeform out there.
Point is though, we're just talking about probability which is always inabsolute, so it's rather a moot point to be talking about whether or not we know there is other life when we cannot know. I think there is other life somewhere, but again, it's based off inabsolute data.