As you've discovered, the key is the display of hidden text. There is no button for turning it on or off. You could use Ctrl+D followed by Alt+H followed by Enter.

Thank you, Charles:
Right now, the only way I found I can turn it on is to make an arbitrary index. After the code XE is on, I can turn it on and off by using the show paragraph sign in home manual. However, it is odd if there is no way to turn it on in the first place.