As an Administrator, you need to go to Reports>User Reports (1) and click on the name of a learner.
Move to the Courses tab (2) where you can see all the courses that the learner is currently enrolled in. On the status column, the progress of the learner is displayed and by clicking on it (3) you will see, in detail, which units are completed.
When you move your mouse over a unit's title you will see additional information regarding how much time the learner has spent to complete the unit as well as the date and time of completion (4).
You can reach the same information, if you go, as an Administrator, to Course reports, select a course (5) and move to the User's tab (6). There, you will find the status column for each user and by clicking again on the percentage (7) you will be able to see all of the completed units.
The infotip will appear once you move your mouse over a completed unit (8).