The Greeks knew the Earth was round because they could look at ships appearing over the horizon and observe that the tops of their sails would appear first. Eratosthenes was further able to calculate the radius of the Earth to a decent first approximation simply by using the shadows cast by two sticks at reference locations. If they could do it, you can do it too[0].

The idea that you "cannot verify" is a very pedantic comment, and using flat Earth as a basis only makes it more comical. Of course you can't know anything "with absolute truth", but nobody cares about that. The relative distribution of evidence strongly disconfirms some hypotheses, there is already a culture of strong distrust and independent verification in the hard sciences (see the LK-99 saga), and you can rely on the long-term output of that process in the same way you can rely on your GPS to just work without needing to independently launch your own satellites. Needing to be your own scientist now is like needing to be your own farmer: completely unnecessary for most people.

[0] Yes, flat earthers claim these results are spurious because of optical illusions caused by hot air or the like. But the relative distribution of the evidence for _that_ hypothesis is pretty darn slim, which only furthers my point.