Well I find there are basically two ways of knowing something:
1. Do-it-yourself. Eg. burning your hand on the stove, you KNOW through your perceptions that the stove is fucking hot, and not to touch it again!
2. Told-by-another. Eg. your teacher telling you facts about some subject or another.
Simply put, 1. is a lot more reliable then 2. That being said, 1. is also not without its flaws, your brain may misinterpret your perceptions, or your sensory organs may be sending you false readings (like when you smoke pot). But for the most part you can trust your perceptions and the information they tell you. Whereas a lot of information from 2. should also be verified either by method 1. or by numerous methods of 2.
The dilemma at hand: 1. is much more time consuming then 2., we don't have enough time to prove everything ourselves, that is why we resort to 2. So the next dilemma becomes how to validate 2. and mostly this is done by numerous validations by 2. indirect validations by numerous 1. or the good old do it yourself 1.
Long story short, we can not TRULY know anything, but we can be REASONABLY SURE, and the closest to TRUE KNOWLEDGE is direct validation by 1. Everything from 2. you should not believe blindly.
I'll just give you a couple funny examples before I go:
Back in the medieval times: everyone KNEW the Earth was flat, every KNEW the Earth was the center of the universe. They all KNEW this through 2., because it was next to impossible to prove with 1. until much later.
Its just funny to think, I wonder in 200 years time, what will we KNOW then?
If anyone has any disagreements with my theory please comment I welcome it and always willing to look over things, after all we are humans we make errors, and we don't KNOW anything for sure!
None.