You can check which workouts you’ve already finished directly in the workout library.
Open the workout library
From the bottom tab, tap Explore and make sure Workouts is selected from the top menu.
Identify new workouts
If you’ve never completed the workout, the button at the bottom will say Start and the complete icon in the top right will not be highlighted.
Identify completed workouts
If you’ve finished the workout before, the complete icon in the top right will be highlighted and the button at the bottom will say Restart.
Note: To see workouts marked as complete, you must tap Complete when you finish a workout.