You know that you know X because your perception, and its consequent interpretation lead you to believe X, and that it would contradict your experience to deny X. I know (!) that this is circular.
For clarity, I believe that belief (circular much?) is involuntary.
The interpretation is the key (interpretation is not necessarily a voluntary act either) – there is plenty of evidence that, cultural biases aside, we really do perceive the world in highly similar ways: I see an apple fall, a Pirahã speaker sees it fall. Therefore, if I perceive X, I can be pretty (though not completely – if you’re seeking a definite epistemic answer for this question, you’ll be waiting a long time) sure that X actually happened. Therefore, I know it happened.
Precisely what happened is open to interpretation.
The interpretation of the event is key. Apart from a dogmatic insistence on a verificationist approach to truth, we must be pragmatic.
If we are to look at the problem from a foundational perspective, I know the proposition “Scott (me) exists” to be true because it would be illogical to believe otherwise.
I think your choice of words in your question actually points to the answer – as I understand it (from an article I read recently on sciencedaily.com, although I’ll be buggered if I can find it now), the regions in the brain involved in determining that a chihuahua is a dog, as is a St. Bernard, are essentially the regions of the brain which are involved in decision making.