If you don't normally keep the Book Details panel open (I don't), here's how to create a custom column to show books identified as available at your library by this plugin.
1) Create a custom column with a lookup name of 'at_library' and a Column heading of 'Available at Library'. Column type should be 'Column built from other columns', and the 'Show checkmarks' checkbox should be enabled.
2a) If you want the checkmarks associated with a particular Overdrive library, enter the following in the template field:
'ocls.lib' should be replaced with the first part of the web address of your library. You can see these addresses in the Customize dialog of the plugin. They are unique for each Overdrive library.
2b) If you want the checkmarks associated with any discovered Overdrive books, enter the following in the template field:
3) 'Sort/search column' should be set to 'Yes/No'
After creating the custom column, you will need to restart calibre for the changes to take place.
After restarting, any books identified as available from your local library by the Overdrive Link plugin will be shown with a checkmark.