From the menu bar, click on Tools, Options, click on the View tab.
Under "Formatting Marks", click on the "spaces" checkbox. In you
document, all spaces will now appear as a small black dot. Double
spaces will appear as two adjacent small black dots. These are visible
only on-screen (they won't print).

If you just want to highlight just double spaces (and nothing else),
you might try this: From the menu bar, click on Edit, Find. In the
Find dialog box, click on "Highlight All Items Found In". In the
"Find What" text box, enter two spaces. Click on "Find All", and
all double-spaces will become selected. So long as you don't reset
your cursor or do any other editing operation, you can step through the
entire document and see them all by using the Page-Down and Page-Up
keys on you keyboard