Genuine question, just curious how anyone firmly on the "schooler" or "unschooler" side of the fence can know without some kind of external reference, particularly one which they themselves didn't choose in case of bias.
Which is perfectly fine as long as you know what that "it" is, which assumes a base-level familiarity with subject X.
The problem cases I can imagine are ones where you don't have the base familiarity, or in other words the cases where you don't know what you don't know (as opposed to knowing what you don't know and where to look for the answers).
This is the case that I see leading towards the so-called "expert beginner" stage of understanding.