In the norther hemisphere even the most casual observer is generally familiar with at least one constellation: Ursa Major, the Great Bear, aka "The Big Dipper" in the US or "The Plough" in the UK. People are also generally familiar with Polaris, the Pole Star or North star. You might even remember that the last two stars of Ursa Major point at Polaris, and Polaris is always within a degree of true north, so quite useful for navigation.
Far fewer people in the northern hemisphere are familiar with the appearance of the skies in the Southern Hemisphere. This is a likely explanation as to why most successful flat earth promoters are in the Northern Hemisphere.
In the Southern Hemisphere there's no easy equivalent of Polaris, as the southern celestial pole (the point at which all the stars seem to rotate about) is in a region of the sky with few bright stars. There is however an equivalent of Ursa Major, the constellation known as Crux, or more commonly The Southern Cross. Crux give us a similar useful locating tool for the southern celestial pole, as the longer arms of the cross point towards the pole.
Another thing that people in the North don't often think about is that in the South the stars seem to rotate clockwise around the celestial south pole, whereas in the North they rotate counter-clockwise.
This all creates an excellent demonstration of the shape of the Earth.
Consider two places on the globe, Sydney in Australia and Santiago in Chile (any similar locations will do). Now in the southern winter the nights are long, just before sunrise in Sydney the sun has set in Santiago. If people in both places look South (i.e. away from the North Pole) at the same time, then what do they see in the sky?
Here's the view from Santiago. Crux is at the bottom.
Here's the view from Sydney,
at the exact same time!
Clearly, despite looking in nearly exactly opposite direction (on a flat earth), we are actually looking at the same piece of sky. This is only possible if we are on a globe.
This is inarguable proof that the Earth is not flat, so if you ever get into an argument with someone who claims to believe in the Flat Earth (and a lot of them, of course, are just trolling), then ask them to explain this problem first to save time.