In both D-spin 7s and Cork 7s you spin 720 degrees.
When you're going to try to cork your spins I would just go for it, pull your left shoulder down and also look down towards your shoulder (if you're spinning leftside) and commit. I wouldn't guarantee thinking "i'm going for a D-spin" the first time you try it. If your first cork becomes a Dspin, well cork less and you have a cork.