When it comes to fine-tuning your site's user experience, the last thing you want your site visitors to see is this:

If you've recently launched a new site, it's always good to redirect your old URLs to the new ones but sometimes URLs slip through the cracks and the 404 page will get visited from time to time.
Below are the steps on how you can use Google Analytics to run a report to see which URLs are generating those errors.
The first step is to make sure that your 404 pages have a standard message in your page title that tells you when an error is hit:

If you have a page title set for your 404 pages, you can find more information about them in your All Pages report located below Content > Site Content. Once there, select "Page Title" as your Primary Dimension and search for the title you identified:

Once you've found the correct page title, click on it to reveal the list of URLs that generated those 404 errors. From there you'll be able to identify which pages need redirects put in place.
Don't forget to use the new Shortcut tool so that it's easier to find this report in the future!