Jim S. mentions a useful proof about the knowledge of sphericity of the earth. The existence of the globus cruciger.
The ball was supposed to represent the earth, with the cross on top representing Christ's dominion over it, and the sovereign would hold it to show that "he's got the whole world in his hands." The earliest of these dates to the fifth century, before the fall of Rome, and they were used throughout the Middle Ages. In fact, orbs without the cross were common for centuries beforehand. Thus, any claim that the ancients or medievals thought the earth was flat can't even get started.Representing the earth with a ball because the shapes match seems fairly convincing. The scenes depicted on some globes make a celestial interpretation of the globe less viable.