Black side is also empty. Better to try to develop film from empty holders than to shoot film that isn't there
Even though I'm apparently 'wrong' with the way I mark my shot and unshot sides (not that it matters, I never learnt from a class and don't work with others so noone'll mess up my film), I've got an easy way to tell if it's empty (you know, besides putting it in the dark bag and feeling around).
When it's loaded (shot or unshot), the hooks preventing you from pulling the darkslide are in place (also helps you stop rushing, slow down and check that everything else is in place, like shutter closed and cocked).
When the holder is empty, the hooks are in the 'open' position (and the darkslide is in the 'unshot' way out). Also, I have different shelves in different rooms for loaded and unloaded (4x5, polaroid, and rollfilm backs).