A) God told him
B) he was a smart guy and observed the world around him
C) he talked with someone from Greece, what would become Persia, or a Chinese fellow.
I'm fine with any of the three, or some combination thereof. The point is by 1492 when Columbus sailed the ocean blue, no educated person thought the world was flat. The idea that they did was put forth to discredit pretty much everyone from that era as being simple fools who blindly listened to the Pope.