If you are registered in the verified track and earn a certificate, this certificate is your proof of completion. You may share this with others to show that you completed the course.  

Once you begin an edX course, on your dashboard the View Course button will change to Resume Course. This button is there for your convenience to return to the course and it doesn't indicate completion or non-completion.

To view your grade towards earning the certificate in the verified track you can view your course grading on the Progress tab within your course. 

Course Completion Chart

This radius chart available on the New Progress page experience represents how much of the course content has a green checkmark. That's not just graded assignments, but also readings, videos, or practice assignments.

This Completion chart doesn't affect your certificate eligibility - check that you've passed the course to assure you're all set. Why doesn't the Course Completion chart show 100%?