How to find broken links on a page, and which ones really are broken

1Check every link on this domain from the Console 1 minute

Open the page, open DevTools (F12), go to the Console and paste:

Copy// Checks every link on this page's own address; lists the ones that fail or redirect.
// Skips logout and sign-out links, which a request would follow like a click.
const links = [...new Set([...document.links]
  .map(a => a.href.split('#')[0])
  .filter(href => href.startsWith(location.origin + '/') && !/log-?out|sign-?out/i.test(href)))].slice(0, 200);
const found = [];
for (let i = 0; i < links.length; i += 4) {
  await Promise.all(links.slice(i, i + 4).map(async url => {
    try {
      const res = await fetch(url, { cache: 'no-store', signal: AbortSignal.timeout(10000) });
      res.body?.cancel();
      if (!res.ok) found.push({ url, result: res.status });
      else if (res.redirected) found.push({ url, result: 'redirects to ' + res.url });
    } catch (e) {
      found.push({ url, result: e.name === 'TimeoutError' ? 'timed out' : 'no response, or redirects to another site' });
    }
  }));
}
console.table(found.length ? found : [{ result: 'All ' + links.length + ' links answered OK' }]);

You get a table of every link on the page's own address that failed or redirected, with its status code or final URL. It sends four requests at a time and stops at 200 links, so it's gentle on the server.

It follows every link the way a visitor would, so run it while signed out, or on a page without links that change things (it skips logout links, but not "empty basket" or "delete" links). It only checks links on exactly the same address as the page: www.example.com and example.com count as different. It uses ordinary GET requests, because some servers answer HEAD requests with a 405 even when the page works.

2Find where a broken link sits on the page 30 seconds

A table of URLs isn't much use to whoever has to fix it. This outlines every link to a URL in red and scrolls to the first one, so you can see whether it's in the content, the navigation or the footer:

Copy// Highlights every link to a URL and scrolls to the first one
const target = '/summer-sale';  // the broken URL, or part of it
const hits = [...document.links].filter(a => a.href.includes(target));
hits.forEach(a => a.style.outline = '3px solid red');
hits[0]?.scrollIntoView({ block: 'center' });
console.log(hits.length + ' link(s) found', hits);

3Check links to other sites a few minutes

The Console can't do this for you. Browsers stop a page from reading another site's responses unless that site allows it, and almost none do. You'll get an error or an empty answer for links that work perfectly well.

1. List them: [...new Set([...document.links].filter(a => a.protocol.startsWith('http') && a.host !== location.host).map(a => a.href))]
2. Open each one in a new tab, or paste the list into a crawler that checks external links.
3. If a crawler reports 403 or a "verify you're human" page, but the link opens in your browser, it's working. That's the site blocking bots, not a missing page.

4Ask Google which links it found broken 5 minutes

For the whole site rather than one page, use Google Search Console: Indexing → Pages, then open Not found (404). Inspect a URL there: under Discovery it may name a page that links to it. This lags behind your site by days or weeks, so use it to find patterns, not to confirm a fix.