This is something written in the manual, but I have noticed that few people know about it, and even less read the manual. So I thought it would be a good idea to write about it.
When you are in manual focus mode, if you half-press the shutter button, the camera will beep and highlight the squares that are in focus. This is a great way to get a confirmation when you are not sure if you are in focus or not. Of course, it cant be trusted at 100% (else why would we use manual focus in the first place?) but it can be useful.
The best way to use this trick is to activate all autofocus points so when one blinks you know where the focus is in the frame.