The easiest way to tell if one of your students is registered is by checking their Role. Go to the Manage Roster page and check the Role column (the last column on the roster). If a student is registered, their role will show as Student. If the student has not registered, they will remain on your roster as pending and their role will show up as Invited. You can filter your roster by role as well to quickly see which students have registered and who hasn't!