I needed to check an issue in which the deep integrated bbPress forum pages were showing up along with a 404 header code generated by WordPress. I am showing you how you can view html headers in both Firefox and Chrome.
HTML headers in Firefox using Firebug
You will need Firebug addon.
Open the page for which you would like to examine HTTP headers.
Press F12, shortcut key for Firebug.
Click on Page Speed, then click on Resources and then on the left, click on the + sign of every HTTP request you would like to examine.
You can view the header code without expanding the request.
HTML headers in Google Chrome
In Google Chrome, open the page for which you would like to examine the HTTP headers.
Press Ctrl + Shift + i, shortcut key for Developers Tool.
Click on Resources, then click on Documents, select the resource and click on HTTP information, Request Headers or Response Headers to expand them.
Now this post may not be handy but it will help for sure when you want to look out for 404 errors being returned on your site. Happy digging around