How will I know when Magic Mouse is fully charged?

If the Bluetooth menu is visible in the menu bar you can use keyboard shortcuts:

1. Press Ctrl-fn-F8 to bring the focus to the status menus
2. Use the left or right arrow keys to select the Bluetooth menu
3. Use the down arrow to select the Magic Mouse entry
4. Use the right arrow to expand the submenus and see the Battery remaining

Click on the Bluetooth icon in menu bar, then select (highlight) your Magic Mouse.

If you don’t see the Bluetooth icon, enable it in System Preferences > Bluetooth.

Except you can't do that while it's charging...