There are a number of ways for finding the broken internal links but that's already identified. You have control over the internal links of your website. So you can easily fix it and you must fix it because that will affect your SEO.
The most common cause of broken links is mostly TYPOS, check if the 404 ERROR is due to the wrong spelling then fix it.
In terms of SEO when the work of that page is worthy then recreate it if the broken page backlinks direct to it.
For making the page working in terms of SEO, use the 301 redirect method.
The best method for dealing with this is deleting the broken link if that isn't crucial to your site.