Like reading a novel, the Iliad, Gilgamesh, etc..
To discern means more than just to view. It means to understand. How does someone understand something he doesn't believe exists? How do you know, discern, understand what you say isn't there?
Your answer is that it's like any work of fiction - you read about and thus you "know" about it and you "know" enough about it to dismiss it as unreal.
But that's third hand. You're only "knowing" what someone else is telling you about something. You cannot know for yourself since you say it doesn't exist in the first place.
So, how do you know?