A broken backlink is an incoming link pointing to an “error 404” page of a target website.
Within Ahrefs Site Explorer, we have a "Broken backlinks" report. It shows all the referring pages that link to 404 pages of a target website along with their anchor and surrounding texts.
Also, you will be able to see some useful metrics like Domain Rating and URL Rating of a referring page, the number of social shares it has, etc.
You can find a full list of all 404 pages that a target website has in another report of Ahrefs Site Explorer called “Best by links”. Just apply the “404 not found” filter.
So that you don’t miss out some valuable “link juice”, it's recommended to restore those broken pages or use redirects to other relevant pages of your website.
The same way you can find your competitors’ broken pages that have a lot of backlinks and use it for your broken link building strategy. Just reach out to website owners who link to 404 pages of your competitors and offer your relevant content as a replacement.