My opinion here is that everyone here is just being an idiot.
What Flat Earthers are saying is that there is no definitive proof that the Earth is round. What you really mean by that is to question the nothing that anything can be proven or truthful at all. And the answer is that it can't, and that all we can do is to make our best guesses based on what we can sufficiently estimate as being true.
The basis of science is to come up with theories which match the evidence, or to find evidence which matches theory.
Flat Earth and Round Earth are opposing theories. There are common elements however, and as always, there are differing elements.
Common elements:
1) feeling of a downward pull towards the Earth
Differing elements:
2) in Flat Earth theory, you could go into space if you kept moving either forwards or backwards
I propose two tests which will answer the question once and for all.
1/First test) answer two questions [accuracy of test 4/10]
First question: how large is the north pole?
Second question: how large is the south pole?
2/Second test) talk to people [accuracy of test 2/10]
a) ask someone from france who knows where germany is
b) ask someone from india who has travelled into china which direction china is
c) ask someone in china which direction america is in
d) ask someone in america which direction europe is in
If Flat Earth is correct, then there should be a curve either to the left or to the right
If Round Earth is correct, they should be able to point to relatively straight points
3/Third test) observation [accuracy of test 10/10]
a) look into the direction of the north pole. you should be facing the center of the Earth
b) look into the direction of the south pole. it should be both beyond the north pole in front of you and it should be behind you at the same time
Please report your results, especially of the Third Test, whether they support Flat Earth theory or Round Earth theory. All feedback is welcome whether it's abuse and slander or whether it is supportive and complimentary.