How do I find broken links?
Click on mentioned hyperlink to know more about how to find broken links -
Many paid and unpaid SEO tools are there to find broken links of site such as google analytics, Xenu, W3c link validations etc. You can choose any of them.
https://www.screamingfrog.co.uk/ is what I often use for these cases. It's free for up to 500 pages.