"...we may not know the shape of the world."
People have been studying the seismic shivers of the world for millennia. The understanding of the reflection, refraction, and rate of travel of those vibrations has become quite advanced lately. The generally spherical shape of the Earth is confirmed by seismic research, as well as much else.
You could find out what the shape of the world is fairly easily, if you discarded false hypotheses. That is the method I recommend.