Find all empty fields in admin
I can't think of a super-easy way to figure this out off of the top of my head.

There are a couple of not-so-easy ways to figure it out, though.

1) Find the column in the craft_content (or the equivalent Matrix content table) and find all the rows with empty values.

2) From either a plugin (PHP) or a template (Twig), write some code that grabs all of the entries you are interested in and loops through them checking if the particular field has a value or not.