There are several 'clues'... The curvature of the horizon. The shadow the earth casts on the moon during an eclipse.
Have you ever heard any news about people falling off Earth's edge? yeah but that doesn't really answer the question.
Because people have seen it from space and it was round also people when they sailed around it they didn't fall off.
Because an atom is round ;).
Because people have seen it from space and anyway if the Earth was flat all the water would fall of the edge so then we would not survive
People are small on a big sphere or earth. If you were on Google maps and zoom in, you would see that big looking city's to you are small on earth and that humans are even smaller. Were so small that...