There is internal evidence within the Quran that Muhammad was not its author. Several verses criticized him, and were on occasion strongly worded. How can an imposter prophet blame himself when it may run him into the danger of losing the respect, perhaps following, of his followers?
How could he allow himself to appear abandoned by God, endangering his very agenda? How could he risk for people to indulge in doubt about his claim? If he was fabricating verses at whim from the beginning, he would produce them in a rush when his relationship with God was being questioned.