From the Full Grade Center place your cursor over the student's first name (or last) and click on the gray arrow that appears to the right. Then click View User Statistics.

Once you're on the User Statistics page look for Items Completed. This shows you how many items (i.e., assignments, quizzes, tests, etc.) are in the course and how many of those the student completed. In the example below the student has completed 21% of the course or 10 of the 48 items.