1. The moons shadow can cue Earth's spherical shape, hence the lunar eclipse.
2. The way we see boats 'rise' from the ocean when looking from a shore. If the Earth was flat, we would see it as just appearing,
3. The field of view considering star constellations would differ on a flat planet.
4. Shadows wouldn't move, but appear in the same location on a flat planet.
5. Seeing farther from higher can explain a phenomenon of curvature.