Besides the Bible, how do we know God exists?
Good question! Because we don’t want to believe in something made up, do we?!
Writing to the Christians in Rome in the first Century AD, the Apostle Paul says that we can tell that God exists, and learn a little about him from the created world, “God’s invisible qualities - his eternal power and divine nature - have been clearly seen, being understood from what has been made” (Romans 1:20)
Paul’s saying that certain qualities or attributes of God (his power and nature) can be seen from what he’s made. Because the world is good, and complex, and ordered, we learn that God is good, and he is greater and more powerful than us, and that he is ordered.
If we were walking through the jungle and we found and iPhone XS on the ground, we wouldn’t automatically assume it had naturally formed without any intervention. An uneducated or primitive person might think that, but we would know there has to be someone behind it, and we would be able to tell that they know lots more about stuff than we do, because you and I couldn’t make something like that! That’s Paul’s point. We can tell something about God from looking at what he’s made.
But let’s not dismiss the Bible in this question. The best information about God is from the Bible. Creation tells us a little, the Bible gives us the complete picture because it shows us Jesus who claimed to be God come to be with people. We can be sure God exists because Jesus is God, and the eye-witness evidence of his life is extraordinary.