Aside from all the experimental, direct evidence of a round Earth (and by the way, no, Earth's shape is NOT a geoid, that is too a simplification), there's a plethora of indirect evidence coming from geology. Essentially, if Earth was flat geology as a science would be entirely incorrect and so would be all its predictions. They're not, however, which proves the obviety of Earth's roundness.

Details are a bit technical but I'd be glad to explain if anyone's curious.