Course Link Validator
The Canvas Link Validator checks for broken external links in your course and creates a report of problematic links for you to fix.
Though the Validator has limitations, it still provides a useful check far more efficiently than you could perform by hand.
To perform the check, use the "Validate Links in Content" link in the right sidebar of your Course Settings page.
Step-by-step instructions and details can be found in the "How do I validate links in a course" Links to an external site. Canvas Guide.