In today's tasks list on the watch you can mark a task as completed and also view tasks that have already been completed today.
Completed tasks in SAU on Apple Watch
You can mark tasks as completed on the watch, in the mobile app or on the web portal. It will automatically synchronize across all devices. Completed tasks will be shown in the list until the end of the day.
You can always see which tasks have already been completed and which still need to be done.