You can see a list of all 3rd party apps that have been authorized to access your TrainingPeaks account by logging into your account and navigating to - https://www.trainingpeaks.com/account/manage-applications. 3rd parties that have permission to sync completed workouts or metrics to your account (Garmin Connect, Zwift, HRV and body composition apps, etc) will have "Write" permissions. Apps that can view your planned structured workouts or completed workouts will have "Read" permissions.
From this page, you can also remove permissions for those 3rd parties to access your TraininingPeaks account.
Note - If you are a coach and also have an athlete account make sure that you are logged into your athlete account when navigating to the Manage Applications page to see which apps are connected to your personal athlete account.