We can't know what we don't know at this moment, but we can always learn.
No, we can't be sure we know what we know. Our brains can deceive us.
Would It surprise you to know that many of our memories didn't happen exactly the way we think we remember them?
Our brains have a lot to do - and do quickly - with only a little bit of power to use doing it (think powering the entire city of Las Vegas for a month on just one AAA battery). To accomplish this, por brain takes "shortcuts". One of these "shortcuts" is with our memories....
Our brains prune our memories. In order to store vast amounts of information, our brains compress memories. During the compression, some small details are removed to "save space" (such as a blue sky or a green tree - things that are relatively constant). When the memory is recalled, the brain fills in the "missing" information with what's already known. In other words, since the sky is always blue, saving 1 copy of that info is more efficient than saving it umpteen billion times. Think of it like this - memories are stored in black and white, but when recalled, the brain adds color from what we already know (the sky is always blue, trees are always green, etc.)
Our brain doesn't have resources to fact check everything, so it's obliging. If we tell ourselves something long enough, our brains will assume that it is. For example, if we tell ourself a lie is a truth, eventually we will believe that lie is truth. (trump is a good president… trump is a good president...)
So, what we think we know may not be actually be what we know...or even knew.