Gravity is the biggest indicator. That gravity always pulls directly down no matter where you stand on the earth is the bigest indicator that the world is round, nothing else would make sense. If the world were flat on top and round on the botton, gravity would constantly continue to pull you into the center, meaning its effects would not be distributed evenly across an uneven surface. No math, just basic fucking sense. =/
NASA would like to have a word with you.
Gravity pulls almost straight down but not exactly. (The problem is a lot worse on the moon--and will bring down just about any satellite placed in lunar orbit. The lunar modules were abandoned in lunar orbit--and aren't there any more.)