You have to look at the ring in the center of the disc. There’s tiny print that says “SIDE 1” or “WIDESCREEN”, as well as the disc’s title, copyright date, etc.
One point in favour of having two seperate CDs/DVDs is that if your player holds more than one disc at a time (or if you have more than one player) you don’t have to go over to the machine and turn it over half way through.