It doesn't really matter where I learned something for the purposes of the specific question or whether it would be considered true or valid by some external standard. What matters is that I know where I learned it. If I don't know how I know what I know, then there is a good chance I picked it up as simply a given. That would make it either a habit or an unexamined norm....
So, if I can't answer the question "How do I know what I know?" then I know it is time to give some thought to that piece of information--some skeptical and critical thought.
Takin Up Space: How Eating Well and Exercising Regularly Changed My Life by Pattie Thomas.
I suggest some categories for knowledge, not necessarily in order:
What you know by experience
What you know by extensive experience
What you know by study
What you know by studying more than one point of view
What you know by studying more than one point of view and allowing that more than one of them might be right
What you know that you've checked sources for
What you know by poking around and gradually accumulating information
What you know because you googled it a little
What you know which would affect your income, social connections, or reputation if you thought differently
What you know because everyone else seems to think so
Here's a study of what some high school students know about the Viet Nam War. It's an effort to find out what's actually going on instead of doing a little superiority dance about young people's ignorance.
The link above is thanks to Chris Williams in a thread at Crooked Timber which is half about Iraq and 9/11 and half about Led Zeppelin. And I found the CT thread thanks to ozarque.