I have developed a whole collection of useless skills. Some people teach themselves to play guitar or write computer programs. I taught myself how to differentiate between a plain M&M and a Reese's Pieces without looking for the little M.
I had mixed a bag of Reese's and plain M&M's in my candy jar. (I don't like Skittles all that much.) Whenever I pulled out a green, red, or blue piece, I knew it was obviously an M&M. There was always some doubt whenever I got a yellow, orange, or brown piece. But after much trial and error and one soon-to-be-root-canal, I can tell what kind it is based on the slight difference in the shade of the color. And the Reese's Pieces are on average .36mm smaller in diameter.
Of course, this serves no purpose. None at all. This skill would be unnecessary if they just printed the little M on both sides of the candy. Or if I just buy the peanut butter M&M's next time.