I saw this a while ago, and was going to tack it onto the next blog post I write. There is something of a noticeable difference between the backs of foil and non-foil cards, to the point that I can separate my foils from my non-foils using only the backs. I'm not 100 percent accurate, but I can spot the difference most of the time. The foils have a richer black color, whereas the non-foils are a smoky gray. If you hold one of each next to each other, you can tell the difference rather easily.
In girum imus nocte et consumimur igni