Here’s a quick tip for you. Google Webmaster Tools has really grown. Yeah, there’s still some squirrely reporting (like why my impression count is exactly the same every day), but the Crawl Errors function is vital for anyone who adds and removes a lot of pages, or has switched sites and URLs.
My company recently got a new website. More than a reskin, 98% of the URLs had changed (for the better). With Screaming Frog and some insight on what the URLs were going to be, I was able to whip together a good .htaccess file to use.
The new site has been live for a few months now, and despite thinking I had the 404 issue pretty covered, I logged into the Crawl Errors tool in Google Webmaster Tools.
I thought I had it under control. Clearly not. But Google makes it easier than ever to fix. Click the Not Found button, and take a look at the list of 404′s it gives you.
Ideally you can clean these up with a couple sweeping server redirects. In my case I simply forgot to remove an old XML sitemap. But the beautiful thing is that each resulting page can be clicked for more information: