A flat earther would tell you the "satellite" photos are faked, and could provide you with faked photos of their own.
The question is how you distinguish the two: always go mainstream? Always counter the old-fashioned ways? Trust the best credentialed person? Distrust credentials achieved during brainwashing, go with independent self-taught thinkers? Independently verify every claim, replicating experiments from first principles?
But they can always say anything is faked. They can say that my eyes distort what I see and that math can’t be trusted either.
The only way you could really prove it is to define a set of axioms that you some how convince them to agree to. And then get them to agree to logic. And then construct an experiment using those rules whereby if the Earth is round then they die. And see if they are willing to test it. And if they don’t then you win. And if they do it and die then you also win.
The question is how you distinguish the two: always go mainstream? Always counter the old-fashioned ways? Trust the best credentialed person? Distrust credentials achieved during brainwashing, go with independent self-taught thinkers? Independently verify every claim, replicating experiments from first principles?