I think it is easy to prove if the Earth is flat. Just fly in a plane to the edge of the world. Take a video. Why has no one done this yet? Its much less expensive than flying a rocket into space.

The spherical version of the Earth is easy to prove because it means if you keep flying in one direction you will come back to your starting point. It would be very easy to prove this wrong. For example fly from the US to Japan by going East. And then try by going West. One way would fail if the Earth was flat and both ways would succeed if the Earth was round. If the Earth is flat it means that airlines are lying to us and charging us too much for certain journeys!