You bring up a great question and I'd be happy to give some information on how you can find these inactive contacts.
With this goal in mind, your best tool is going to be Segments. The segment builder is found on the Audience page by hitting the "segments" tab in the grid view of your groups. You'll want to make a segment based on Response History that tracks members who haven't opened within a period of time you specify. This will pull together the list of contacts who are in your account, but may not be engaged with your mailings. Check out this article on our resource center for even more information on how to use segments to find key data.
I hope this is helpful! Let me know if you are running into any trouble while making that segment.