I thought of this experiment, but I am not sure how well it would work, or if anybody has the proper tools.

Somebody with a good camera and a tripod should find a high elevation right next to a large body of water, and setup the camera perfectly level. Then take several pictures and see where the horizon is on the camera. On a flat earth, the horizon should be exactly at the mid-point of the field of view, but on a round earth, the horizon should be a couple arc-minutes down. I am not sure if anybody has camera's that could detect that, but here is the formula for how many degrees down the horizon should be based on how high off the ground you are (the higher the better):

cos^{-1}(6371000/(6371000+h))=angle

h is measured in meters, and the angle is measured in degrees. At 1 meter up, the horizon should be 2 arc minutes down, but at 50 meters up, it should be about 13 arc minutes.

I am not a camera person, so I don't know if cameras that can see such a small angle exist, but if anybody does know, please say so.