You haven’t answered the question. You haven’t even addressed it.
The question is not “how do you know what you know?”, but rather, “how do you know what another doesn’t know?”
One way: you look at the fruits. You think about what observable things ought to be true if the person has the knowledge they claim, and then look for them.
If someone says that they know how to turn farts into gold but they're perpetually broke, they probably don't know how to do this.
Someone who says they know the secret to inner peace ought not to be making angry rants online. Someone who claims they know the way to achieve eternal life ought not to fear death. Someone who claims to know how to heal the sick ought to be able to do better than chance in a randomized controlled trial of disease outcomes.
Since knowledge is acquired through experience, you cannot, (at least not without calling them a liar) dismiss what another claims to know, unless you have had a similar experience.
Not necessarily a liar; people can be honestly mistaken.
Acquiring knowledge from experience is a three step process:
1. I have experience A.
2. From experience A, I infer knowledge B.
3. Therefore, I know B is true.
Some people who really do satisfy step A absolutely suck at logical inference and go completely off the rails in step 2 and 3 and end up making conclusions that aren't justified by what they actually experienced. The good news, though: absolutely anyone can just take experience A as a given and figure out if it implies B.
All that aside, liars do exist, and charlatans often take advantage of unverifiable claims. If there are strong indications that someone is lying, we shouldn't be afraid to call this out.