First consider if the earth were flat, how would a shadow look. We assume that as the sun moves (or actually as the earth turns) a shadow will reflect this. So putting a stick in the ground and watching it shadow should give us an idea of whether the earth is round or flat.
This test involves placing sticks at several points around the globe and seeing what shadows they cast. If the earth were flat shouldn’t the shadows all be alike. Since the earth would not be rotating as the globe (an orb), the shadows couldn’t move either so the shadows should all be similar. If you think the flat earth does rotate, it could only rotate like a record on a turntable. If the sun is in a stationary position relative to the earth, the shadows again would move but their length would not shorten because the sun’s position relative to the stick would not change the angle.
This is not an absolute proof that the earth is not flat but it is correct in its assumptions that can be drawn.