I found this article on diagnostic styling and it really helped but a friend of mine took it one step further. Instead of red the best color for diagnostics is lime green. You are most likely not going to use it in your styles so it will really stand out on your page.
So follow the steps in this article but remember to replace the color specified with lime green.
If you have any css tips, tricks, resources email them to Susan Fischer at firstname.lastname@example.org and we will post them.