How anyone knows anything at all is the million dollar question, one that can only be answered following an in depth study into the nature of and means by which we as rational beings acquire, integrate and confirm knowledge. I think we are on the right track for embarking on such a quest by virtue of our refusal to simply take someone else's word for it, especially someone who claims that it simply cannot be done, that one cannot be certain of anything, as if that was somehow a sacred fact, not to be disputed, while oblivious to the fact that they just refuted their own assertion.
If there is one thing of which I am certain it is that certainty cannot be obtained by simply listening to and believing what we've been told, no matter how reliable a source of truth someone might have proved to be in the past. Certainty is derived from within through our own understanding based on and to the extent that asserted facts correspond and correlate to ones own personal observations and experience.
When it comes to asserted knowledge, whether or not it is truly factual, apart from ones own understanding, there's no way of knowing, good or bad, right or wrong, what you might be missing. While no one can claim omniscience, the biggest fool of all is the one who refuses to acknowledge their own ignorance.