http://www.craftyarncouncil.com/weight.html
While it's technically considered a "4", I've always considered it sort of a heavy worsted. If you look at the link above you'll see that worsted weight/aran varies from 16-20 stitches over 4 inches. That's a pretty big gauge difference.
Laura - After awhile you can usually look at the gauge on the yarn and have a pretty good idea what weight it is.
