How do I know what I know?

I know the things I know basically from what has been taught to me or things I have experienced firsthand.  If we don't see things or witness things for ourselves, I guess we don't really know anything but as humans we assume and trust other humans to what they say or know, even though we may not know for certain.  I think that if humans only believed things they have experienced, then the world we know would most likely be a completely different place.  For all we know, if people didn't take leaps of faith in trusting other people, the world could be centuries behind in technology or advancement, or if more people trusted others, we could be centuries ahead of our time.  This is a very speculative discussion that many may have trouble understanding, but I think that as humans, the majority are wise enough to decipher and figure out what is real and what isn't