On the web:
- Under “Add a new task…” click the “Filter: Uncompleted” button.
- Select “Completed” or “Everything”
On the Android:
- Press Menu
- Sort & Subtasks
- Check the “Show Completed Tasks” box
- Press “Always” or “Just Once”
On the iPhone:
- Press the “Active” button on the top right
- Select “Done (completed only)”