I'm not sure where this flat Earth stuff is coming from. The only explanation that makes sense to me is that, since our government is now blatantly lying about most things today, that one will just assume they are lying about everything, including the Apollo space program and the Earth being a sphere.
Given that, there is a very simple way to prove the Earth is indeed a sphere. Just look at flight schedules of airlines that fly between, say, Bueno Aires or Santiago and Sydney or Auckland. If the Earth was a disk like the flat Earthers believe it is, these flights would be impossible because they would take 30-40 to complete, several times the range of any commercial airliner.