Any sentient being which considers the problems of epistemology will realize that without verification from other beings or greater realms of reality it cannot know if it is correct in it's world view. It doesn't matter how intelligent or knowledgable a being is - if that being wants to verify it's knowledge to make sure that it is correct then it needs to look to something more intelligent than itself.
God is called all-knowing. This means that God knows everything but I believe it can be shown that this is a logical impossibility, and that no being can know everything.
If a God was to sit down and ponder the problem of epistemology and wonder whether it does not everything or not, it would realize that it has no way to verify that this is true. How does it know it knows everything? It merely thinks it does. The God has no test, method or possibility of finding out if it does indeed know everything. In short, it does not and cannot know if this is true. God does not know if itself is omniscient or not. God therefore does not know everything and is not omniscient.