Ecozombie: Heal Thyself
If while on a trip to the South you looked in the sky you might see the irrefutable, that the full moon hanging in space was round. As you traveled southerly it would also become undeniable that mountains to the north were sinking below the horizon and new mountains were arising in the south. From these experiences you could have a surefire thought that this was happening because the Earth was a globe and you were traveling its circumference. In addition, empirical evidence from ships you might see sailing away from you that were disappearing below the horizon would further this arguably indisputable round-earth conclusion.
Noting the incontrovertible curved shadow of the Earth on the moon during an eclipse would provide more corroborated evidence, as would the indisputable fact that the stars were setting in the north and rising in the south as you traveled south.