[BlackFly]

Measure your shadow at solar noon, fly south and measure your shadow at solar noon there. (Solar noon may differ from 1200.)

Essentially, just repeat this ancient experiment: http://www.juliantrubin.com/bigten/eratosthenes.html

[tzs]

That only works if the people you are trying to convince believe that the Sun is far enough away for its rays to be essentially parallel at the two measuring locations.

Do flat Earth believers believe in a sufficiently large solar system for that to be true?