How do they tell the difference? Impurities?

The very cheap artificial sapphires are basically made by melting aluminum oxide in bulk. They're usually cut in a factory, so they don't look very nice. I got very large artificial ruby (also corundum) for my wife (actually to satisfy my own curiosity), and it basically killed all interest in rubies for her. It basically looks like a chunk of plastic. Under high magnification, you can supposedly see a lot of parallel curves lines of bubbles due to how the crystals are grown.

There are also nicer artificial sapphires which are grown in similar conditions as natural ones. These have inclusions similar to natural ones, but they are usually uniform in color. Natural ones tend to have parts that are more saturated. It's not a technical limitation though, so I'm sure you can make less perfect-looking artificial sapphires that can fool your average jeweler. However, gemological testing will use advanced spectroscopy, which spits out enough data to distinguish between natural and artificial ones.