The actual video talks about the method in which you have to prove it (using eclipses etc.), so i think it's not as easy as showing them a boat coming over the horizon and seeing the mast, then the rest of the boat slowly revealing itsself, proving the earth has spherical shape
If you go on a flight Westwards you'll reach your destination.
If you go on another flight Eastwards en route to the same destination you'll reach your destination as well.
If you travel in two opposite directions and still reach your destination you will prove that the earth is circular and not flat.