In the upper right corner of the game window is a black tile that shows one of the following: ), <empty>, (, or O This tells you what the moon is like right now. If it isn't a full moon, you get the kid. If it is full, you get the beastie, and that kid is definitely a were beast of some flavor.
Just remember, if you plan to were-ponize your militia, that different types of were creatures are hostile to each other in addition to everything that isn't a were creature. If your entire fortress is made up of were-beasts then you shouldn't have them killing each other when they turn. But Armok help anything else that happens to be in your fortress at the time.