Heres a simple experiment I just thought of. Using surveying equipment, point a laser in any direction and make sure it is dead level. Then place a reflector as far away as the laser beam allows, angled 90 degrees from dead center. If the earth is flat, the beam should return its origin. If the earth is round, the curve would make the laser bounce back and return higher than its origin, or miss the reflector completely. Do you reckon that would work?