Campy chainring identification?
 

Pardon the dumb, here, but how does one determine the group of a Campagnolo chainring? Some have arms, some have slots, etc. I know that there are 144 BCD and 116 BCD (among others), but there seems to be little marking beyond the "Brev Campagnolo" and the (sometimes) letter "C" in a diamond. Of course, they're generally good about stamping the tooth number on the ring, but if you have a box of rings, how does one determine which is what?

Thanks.