Looking up code connected to a storyboard's button

I have seen how to connect a button in a storyboard view to code that does its action by dragging that button into the assistant editor.

Now I need to do the opposite. There is a button in the storyboard view that is already connected to code. How can that existing already connected code be quickly looked up for a button?

Looking up code connected to a storyboard's button
 
 
Q