(Astronomically) At first glance, there should be no observable difference between if earth is a circle or sphere. In both cases, there is a spinning wheel of stars spinning around the star nearest to end point of axis. Simple enough. However, this should vary depending on if you are standing on southern hemisphere or northern hemisphere. In a 2d circle, the northern hemisphere and southern hemisphere both have in view one axis, whereas in a 3d sphere there would be 2 axis such that only one axis end point (south star or north star) would be viewable as opposed to 2d circle where there is only 1 axis end point.
TEST:
This should be easily testable by having one person stand at south pole, and another stand at north pole. They then should point there cameras to south axis point and north axis point, and post photos on internet for comparison.
EXPECTED RESULTS:
* round 2d world: both photos should show same stars in the sky with north star being focal point.
* sphere 3d world: one photo shows north star focal point with north star neighboring stars, and other photo should south star and neighbors.
Shouldn't this test alone be enough to prove if earth actually is flat or round?