If YOU don't know, and someone is confident in their answer, you can't possibly know if they know for sure or not.
And when someone else who DOES know disagrees, how do you know which one is lying? You don't! You can only go by who SOUNDS more right, and that is often manipulated by what you wanted to believe before the conversation even started!
I've literally told a story of something that happened to me, just to have that person say that it happened to THEM and I'm just remembering the time they told me about it.
Tags like "original source" and "number of iterations from original source"
("Iterations" probably isn't the right term. If Bob saw it, then Bob told Sally, then Sally told Frank. Frank has "3rd iteration" knowledge. But what's the better term?)