How Do I Know What I Know?

Honestly, this is such a broad topic.  Knowledge is the basis of everything.  How do we know what to do without basing our actions on previous knowledge?  We can take this question to so many levels.  Eventually we would be questioning who we are and the nature of our existence.
For the purpose of this blog I would like to focus more on "How do we know that our knowledge is true and factual?"  Think of something we have been told our whole lives that we take to be irrefutable facts.  "The Earth is round".  "The building blocks of matter are atoms".  For the average person, these are phenomenons we cannot observe with our own senses.  I don't know anyone who has been out in space and seen the Earth and were able to tell me they saw the roundness with their naked eyes.  There isn't anyone who can shrink down to microscopic size and see the protons, neutrons and nuclei.  But we take these statements to be universal truths because they have been proven again and again.  There is ample evidence from numerous sources that say so.  We as a society have reached these conclusions through experimentation and inference.  We have even developed tools and technologies that allow us to virtually observe these phenomenon.  And so we can agree, this is true, this I know, this is irrefutable.
But there comes a point at which we must consider grander conclusions.  Statements that are no longer considered laws of the universe, but are just theory.  Though we have accepted that atoms are building blocks of matter, we do not yet know what makes up an atom.  There are several theories supported by results of ample experimentation, yet we have no concrete evidence that proves any at this point.  In addition, we know, yes "the Earth is round", but we have yet to ascertain the nature of the bigger universe.  Is space finite or infinite?  Again, there are theories that are both supported by evidence from what we can observe.  But many of these theories, though based on fact, are not themselves factual.  There comes a point in which knowledge is not KNOWING but rather rationalizing with doubt. 
And so for the purpose of differentiating factual knowledge versus theory, I would say the things that I KNOW to be true are statements supported by observation and public consensus.  If I see pictures of the Earth in all its roundness and there is no argument among the majority of our community on this fact, I KNOW that this statement is true.  But once there is debate among people and my conclusions are inferences rather than solid fact, I BELIEVE in something, rather than KNOW it.  It is a fine line between these two which gets fuzzier and fuzzier the more we stray from strictly scientific and mathematical studies.  Even in mathematics and the sciences it is hard to know when to draw this line.  Oftentimes, I think ours is a subjective world.  There is not much that i KNOW i know.