If either center section is out of the housing, all you have to do is look, there are lots of pics here to tell you what a clutch sg, cone sg, and open look like.
If they are both in a housing, turn the rear end yoke. With a sure grip, both wheels/axles turn the same direction. If its open, one wheel will turn one direction, the other either wont turn, or it will turn the opposite direction .
