What do atheists think of the claim "There is no God" being a negative universal?

In case you don't know, a negative universal claim in the context of God is a claim to the effect of X does not exist. Yet, if it were not impossible for X to exist how could one know that X doesn't exist? If I say a snake is not in this room, I would have to see all of the room and notice the absence of a snake. However, when you are talking about the whole universe which for practical purposes you could not check entirely, how could an atheist say that God does not exist?