Rel=nofollow is a link attribute typically used by webmasters to signify that they do not want search engines to follow and credit a link as related or relative to a specific page or site. If your using the firefox browser, it is simple to detect which links are using the rel=nofollow attribute on any given page by installing the popular SeoQuake addon. This addon enables a user to detect and displays rel=nofollow liks with a strike through or strikeout line (line-through rel=nofollow links). In addition, SeoQuake contains SEO tools that may come in handy. One such tool is the SEOBar, which can display information about any page such as Pagerank, Alexa Rank, Inlinks, Age and Whois.
First of course, if you don't already have it, you'll need Firefox:
How to automatically detect rel=nofollow in firefox: