There's a way to see at least some of what Facebook isn't showing you.
https://www.facebook.com/feed/missed_stories lets you "[c]heck out stories you haven't seen yet" that its algorithm has hidden from you. The Daily Dot did a piece on the feature back in March.
I'm reassured when I see "There are no more posts to show right now" fairly frequently in my Unread Stories feed.
Of course, it might just be Facebook lulling me into a false sense of security...
Hat tip: vvalkyri, in someone's FB comment stream.