To view which workouts you have completed, you'll need to open each workout individually.
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​1. Open workout library: Select Explore from the bottom tab and male sure Workouts is selected from the top menu.

2. A workout you've never completed before: The button at the bottom says 'Start' and the complete symbol remains unhighlighted in the top right.

3. A workout you have completed before: The complete symbol is highlighted and the button at the button says 'Restart'.

Note: You will have to mark a workout as Complete when finishing your workout for this to work.