There is no way to tell if a specific rubric is in use in a course other than manually visiting every gradable assignment/discussion/quiz and checking if the specific rubric is attached.
It would be useful if the Rubrics page or the specific rubrics reached from that page had (at minimum) an indicator of whether that rubric was attached to any activity in the course and (ideally) what activities that rubric was attached to. Then, if I go to delete the rubric, I know where I will need to go put a replacement rubric.
