WebPart collector will help you identify any webpart or all webparts that are located inside your site collection
It basically traverses each page within SharePoint to identify what web parts have been added on to that page.
There are many options that can be selected to view the webparts. You can either select only the custom web parts in pages or web parts added in pages that are in the quick launch.
This would be immensely helpful to identify whether a particular web part is added onto the site collection or not.