I know God exists because I studied his prophecies, and saw them to be 100% accurate, in actual history, not just biblical history. One example is that the Bible says Israel would be scattered but in the last days they would return. These were fulfilled, first in 70AD, and next in 1948.
Because I know the prophecies to be correct, against all odds, I can count on the Bible to be true, as it is not only propheticly, but historicaly accurate too. And the BIble speaks of God, so I know God exists.
I have also had personal experiences which lead me to know God is real, but as that is subjective, I won't tell about them. I have observed how many people's lives are drastically improved once they come to faith in Christ. Alcoholics become productive citizens, etc. so I see God changeing lives for the better, whereas I have seen people go into satanism and have nothing but turmoil and tragedy haunt their every turn. But that is just personal observation.
There was actualy a preacher who invited an atheist to bring 100 men and he would bring 100 men, who's lives were drasticaly changed for the better for their beliefs, the atheist backed down.