I'm positive about it.
When you look at the bottom of the credits page of your books, you'll see this: 10 9 8 7 6 5 4 3 2 1. If it goes all the way to 1, then that's a first printing. If it is, of example, 10 9 8 7 6 5 4, then that is a 4th printing. Both the DMG and the MM are on their 4th or...