Display Widget Areas

I’m trying to figure out where all the different areas widgets might be placed are on the page. Is there a map or diagram of where they are? Or a way to highlight them?
Specifically, I’d like to make use of all the whitespace at the top of the page above the menu bar. But also simply knowing the placement and difference between “Content High xxx”, “Content Low xxx”, “Navigation Trio xxx” etc. would be useful.