There is a system built into 5e to tell you exactly how knowledgeable someone is - you roll an intelligence check (of the type corresponding to the knowledge you are seeing if they possess) to see if they know a thing or not. This seems like a really natural way to reward players for investing in skills that don't necessarily get a ton of use otherwise. And it avoids the very weird notion of "well, my character is very knowledgeable; so if it's a thing that can be known, they definitely know it."