This is an old post. Please check the updated post about the improved version of the "Detect already bookmarked pages" tool here: Improved "Detect already bookmarked pages"
(1) Make sure the feature is turned on (by default it is turned off):
- Click on "Settings" in the main menu
- Turn the "Detect already bookmarked pages" switch on
(2) Load any web page in your browser then click on the "Add to Ninja" bookmarklet or browser extension button.
(3) In the bottom of the Add Bookmark dialog you will see one of the following messages:
- Not bookmarked yet
- Page is already in the Catalog
- Page is already in tab name > category name
Related blog post: Edit already added bookmark in the "Add Bookmark" dialog box