As your website grows it gets harder to keep track of all links. A change you make on your site may break an internal link, and you can’t control what other sites do that may invalidate your links to them. But there are several ways of finding broken links so you can fix them.
For internal links you can look at your web statistics and find the 404 error pages. These errors occur when someone clicks a link that points to a webpage that doesn’t exist. A link can become broken if you deleted a page, moved it to a different URL, or typed in a wrong link. Make sure to regularly check your statistics for 404 errors occurring on your website.
Another way to find broken links is to use a link checker. You can use Google’s Webmaster Tools to quickly identify many errors on your site, including broken links. Another tool and a long time favorite of mine is Xenu’s Link Sleuth, a small Windows program that spiders any site you send it to and returns extensive reports on all it has found, including broken links.
An added benefit that Xenu has is that it can also check external links. By running Xenu regularly on your site you can keep track of your outgoing links and correct them if one suddenly stops working.
However such automated tools have their limitations. Nothing can replace a set of human eyeballs when it comes to checking if the pages you send your website’s visitors to still have the content you want them to see. So go through your site once in a while and click on every link and button to make sure it all works as it should.