Check the assignment list on the user’s profile page. If the assignment status shows “In Progress” the learner has not completed the assignment.
If it is a Course, check the course details. Determine which lesson(s) the learner has not yet completed. Also check if the course has a minimum time requirement. If it does and the learner completed all lessons but didn't spend the required minutes in the course, they are not considered complete.
Sometimes a learner completes an assignment after a notification email is already queued be sent. When this happens, tell the learner that no further action is needed unless they continue to receive notifications.