Well, thanks I guess, but sure he does. Not everything that claims to be scripture is. There are hundreds of gnostic books I do not accept as scripture. I don't accept the Urantia Book as scripture. Just because there can be additional scripture not currently in the Bible, like the Book of Jasher, doesn't mean Maccabees for example, is scripture.
How does one know? That is a the crux of the matter. I don't accept every word of one of our church authorities as scripture. If they were to represent that the Lord told them to tell us something, then I would probably accept that as scripture. If I saw a potential conflict with other accepted scripture, I would study the issue and pray about it.
TBH I'm not sure how Catholics are supposed to know what their pontiff says is scripture or not. Does he say something like "this is the infallible word?" In my studies it seems this is usually determined after the fact...