Tougher than it seems. It's not so bad when you're in a city and there's some kind of writing and architecture, but often as not you're in the middle of nowhwere and it's all guessing based on rocks and trees.
There's other things to look for. Are they driving on the left or right? Is the road dead straight (if so maybe Australia/USA?) Is the road line down the middle yellow (USA and I think SA) or white? etc.