There are a few ways you can easily tell what content you have completed:
The Percentage Bar and Minutes left
Each Learning Item, Course and Mission have a percentage bar that indicates how much of the given item you have completed.
CheckBox to Indicate Completed Content
When you see a check-box next to a Learning Item you will know you have completed it. See the circled images below.
Learning Path on the Home Page
Progress on your current learning path.
To complete a Mission you will be required to pass a challenge. Upon passing a Challenge you will be awarded a Badge. You can review the badges that have been awarded here.