What is proof? The only "proof" that Flat Earthers offer is testimony that is clearly bogus since it doesn't match reality. On the other hand, they deny all the testimony of people who have circumnavigated the globe, in aircraft and by water.
Let's start with Latitude and Longitude.
There's a proof right there.
If the Earth were flat, latitude and longitude would present a polar co-ordinate system and so all distances between two points would not match real life, except for points on the same line of latitude.
Really, you don't need anything other than to understand that, to know that the Earth is a globe.
http://www.indiana.edu/~ensiweb/lessons/flatrth1.pdf