You don’t.
“To know” is a trap, of sorts; in a way, it’s the ultimate trap. “To know” relies on an underlying assumption that whatever reality is, our intellects are capable of accurately modeling it and extracting from those inner models some hard facts about reality. “Knowing” assumes that what is true of our semantic understanding of reality must be true of reality itself.
In this, we are mislead by our limited successes in using our intellectual models of reality as a practical guide to getting things done and predicting outcomes. Because our ideas about how things are seem to corroborate pretty well with what we observe in our experience of the world, we figure that we must be capable of refining those ideas until they are distilled down to some essential truth. When we believe that we have reached that essence, we feel that we know.
Scientists rarely make the mistake of saying that we know this or that in any absolute sense. Philosophers are just as reticent. But we laymen are far more casual. We “know” with an abondon that would horrify any disciplined scientist or philosopher. We feel especially free to indulge our predilection for knowing in the religious domain, especially when we’re given some written record that purports to be the very standard of Truth.
The reason I say that “knowing” is the ultimate trap is that It represents a kind of death of the spirit, a tossing aside of inquiry. When we know, we cease to wonder. It’s an abandonment, really; a refusal to keep asking. There is an implicit assumption in all this that there is an endpoint to inquiry, some kind of definitive answer. But is there?
In the end, all we really have is our current best guess, and we need to come to grips with that fact. When we think we know, we’re just not asking hard enough.