Philipp Lenssen has a nice idea: highlighting every link with the NoFollow attribute easily with Firefox:

A nice way to visualize which links use the new "nofollow" relation is to include a special user-stylesheet into Firefox. To do so, you need to install the Web Developer extension (unfortunately, Firefox doesn't allow an easy way to include user-stylesheets via the native menu). Once you've got this extension running (this needs a restart after installation) you can right-click any page and select Web Developer -> CSS -> Add User Style Sheet. Now you select a simple text file with the extension *.css (you may want to call it "user.css" and put it right on the desktop), in which you saved the following lines:

a[rel="nofollow"]
{
background-color: red !important;
color: white !important;
font-weight: bold !important;
text-decoration: none !important;
}

This will instantly reformat all "nofollow" anchors on the page to have white-on-red colors – no need to check the HTML source for what's happening. And here's a nofollow link to Example.com for you to easily try it.