Here's a test that one could do to find out if the Earth is flat.
Start on the edge of a continent, and sail outwards, or towards the so called "edge" of the earth. If the world is flat, couldn't you sail right off of it, or at least see the gigantic "ice-wall"? I'd like to see someone who believes in the flat earth theory actually partake in this trip.
That test is good, but impractical. We'd like tests that we can do in our own homes (ideally), or barring that, one that doesn't require more than a short drive or hike.
That's not because we're lazy -- okay, it's not
entirely because we're lazy. The real reason is that we want to be able to give anybody at all a description of the experiment so that they can check for themselves.
If somebody performed the sailing experiment and came back saying, "I did it, there was no ice wall," then everybody could just complain that he's lying or crazy or made a mistake someplace, and while that guy would know, he would fail to convince everybody. However, if we had some test that everybody could perform, right now, and get the results right away (or maybe in a couple of days, as in the torsion balance experiment), that would really be ideal.
-Erasmus