Finding Windows keys in use throughout corporation
Microsoft's VAMT will give you an overview of what licenses you have in use and their activation state. It won't break it down to specific keys, but you can use this to determine whether or not you're in compliance.
If you have more than a handful of computers, you really should be using volume licensing anyway, that way you either have a single MAK key, or you have a single KMS key that sits on a server and automatically activates your unkeyed clients. There's really no reason to purchase individual keys. After you hit about 10 or so licenses, it usually becomes a cost-saver. Not to mention that you get to avoid headaches like this.