I think Colombus ansered that question some centuries ago, you can travel too all AROUND the earth and see for yourself...also there are instruments that show as the Earth is a shpere , sooo even if we forget about the millions of satelites orbiting the Earth ( in order to...orbit you need a sphere) there are ancient instruments to prove it.
Ancient instruments like a gyroscope?
If you get a mechanical gyro-compass and take it aboard an airplane and fly straight east or west by this gyro-compass (turn off all on-board magnetic compasses, GPS etc.) you will eventually arrive at the Antarctic ice wall.
Do you know anybody who did this? You didn't, did you? Will he/she testify in court to that effect?
I think we all know that if you fly east or west at the equator, you will ultimately wind up back where you started.
So we need a plane that can fly long distances without refueling and a pilot willing to navigate using a mechanical gyro-compass. However the plane will be flying without a flight plan. What about interference from military aircraft, homing missiles, computer controlled machine gun fire, explosive shells and EMP bursts? That's just to start with BTW, there wont be any witnesses left so who knows what kind of weapon will be deployed.