There is no guarantee whatsoever that the world represented by our senses correspond to an outside reality. Karl Jung pointed out the outside world, as we perceived it, was a creation of the mind, just as dreams were - it's only arbitrarily that most of us believe in matter. Chinese Philosopher Chuang Tse expresses the same idea when he says: Yesterday, I dreamt I was a butterfly. How do I know today I am not a butterfly dreaming he is a man?
Consistency, I suppose is what assures us of reality. But again, the dream world seems to counteract that idea of certainty. When we dream, it doesn't seem strange that we jump from one room to another, from our childhood to our adult life, etc... We don't notice the inconsistencies. So how do you know?
Certainty, of course, is more of a Renaissance to XIXth century philosophy problems. Post modern philosophers have a more open attitude. They didn't feel the need to anchor systems in absolute certainty and had more of a 'Sure enough' approach to reality. That's the continentals. The Anglo-American movement, which was more concerned with science went for the approach of logical and mathematical consistency, but they're more concerned with establishing universal science than assuring a man of his existence.