I learned this as a kid, because I read my mother's advice/etiquette books for young people, and because the Childcraft Encyclopedia volume on "signs and signals" covered it as well. You put your utensils in a criss-cross (X) position if you set them down but are not done. You put them together at the "5-o-clock" position if you are finished, which is a signal for the waiter to take your plate away.

I always do the latter when I eat in a restaurant. It honestly annoys me a bit when wait staff still ask "are you finished with that?" or "may I take your plate?" but then again I know most people aren't trained in that kind of etiquette so it probably isn't entirely safe to assume.