I should start off by saying the only reason I say 99.9% proof instead of 100% is because you should always allow some leeway for a factor that is completely unknown.

An overview of my idea is easily seen when you compare the relative size of Australia to Asia between the flat Earth and the spherical Earth maps.
There is an obvious discrepancy here. Australia is much larger (or at least wider) on the flat Earth map than it is on the spherical Earth.

The length of Australia is something that can be checked though.
There isn't one road going completely horizontally, or I suppose on a curve that is equidistant to the North Pole, going completely across Australia this can be roughly judged without even needing people from Australia to get out and measure it.
It's not something I've done but a brief glance at the road map of Australia the actual length can be estimated for confirming whether it is closer to the spherical or flat Earth.

The reason why Australia needs to be larger on the flat Earth map is to fit with what direction North is on each side of it's coast. That's something that can easily be checked and then the flat theory falls apart.

Quick check on google, width of Australia 3.1km, width of Asia 8.8km.