I'd argue that no being CAN be omniscient, because it cannot know whether it is part of a bigger whole-- for example a collection of multiple beings, isolated from each other, who each think they are omniscient.
And what is an omniscient being who can't know for sure whether it is omniscient? It is a paradox-- it cannot exist. So, there cannot be an omniscient being, even God.
And what is an omniscient being who can't know for sure whether it is omniscient? It is a paradox-- it cannot exist. So, there cannot be an omniscient being, even God.