You can know which items you've posted to Facebook by navigating to the Manage Inventory and clicking the "Links" column.  Clicking the "Links" column will sort the links in ascending order first brining all of the items that were posted to either Facebook or a PopItUp Party most recently to the top.  Click the "Links" column again to resort in descending order.  This will bring all of the items that have not been posted to Facebook to the top.  Depending on the number of items you may want to change the "Show" box to display more items on the inventory page. Also note that unless you change the view from "In Stock Only" this will only display the items that are currently in stock (with a quantity of 1 or more).