You can filter the Find Classes page for classes that a particular instructor has been added. The instructor does not have to be assigned to any sessions or be the primary instructor for the Class or a session. They just have to be in the class(es).
Find Classes
On the Admin menu in the Classes group, select Find Classes.
On the Find Classes page, select the Add filter dropdown and select Instructor. The Choose Instructor dialog will open.
Add the criteria for the instructor you want and select Search.
Select the instructor from the results and select OK.
Select Search.
All classes that are scheduled using the instructor will show in the search results. The Instructor column in the results will show the instructor who is designated as Primary for the class.