How do Muslims know the Qur'an is the direct word of God?

Supposedly an angel told Muhammad what to write. I believe the angel was Gabriel. This revelation happened over the course of something like 23 years. My concern is the proof. Are there any historical doents that prove what Muhammad said is true? In all religious texts we put our faith in God, but we also put our faith in man. Those men being the ones involved in writing the texts and those men who interpreted/spread the word of God from the beginning. So, how do you really know he was visited by an angel? How do you know the Qur'an is 100% accurate? That is a bold statement. Muhammad was illiterate. He had to put his trust in his friends to write what he said was God's word. What if they messed up? Christians acknowledge errors made by man in the Bible, but Muslims do not acknowledge any errors made by man. As if man is infallible.