The simple answer for me is: "Because I spend the day with Him." But that's probably not a very satisfying answer for you.
I started off, as a child, believing there was a God because… well… that's just what you do when you're raised in the middle of the Bible belt. But my thinking changed after leaving home for college and becoming surrounded by people of different backgrounds. One day I shouted out, "God, I don't believe You exist."
I tried other gods for a few years. I dabbled with Zen Buddhism (through a roommate), worshipped the arts, got caught up briefly in the anarchy of the late 60's, and lived in a New Age community—to name a few. After becoming disillusioned with those gods, I began to worship the god of "nothing."
But the pressures of life caused me to eventually cry out in desperation for the God that just had to be somewhere: "I'm tired— I'm done. If you want to do something with this piece of dog sh*t then do it, because I'm finished."
With that less-than-pious prayer my world began to change. Although I've never experienced God Himself with my physical senses, every day I experience the benefits in this physical world of His involvement in my life.
How do I know He's real? I know because He helps me. He keeps me company. He protects me. He heals me. He gives me favor with other people. He gives me creative ideas. He encourages me. Whatever I need, He provides.