Here's a very interesting article based on a patent filed by Microsoft concerning how to detect link spammers:

Do link spammers leave footprints? - SEO and Internet Marketing blog, news and views
http://www.huomah.com/Search-Engines/Algorithm-Matters/Do-link-spammers-leave-footprints-.html

One quick note: I've posted tons and tons of ways to find links on this thread, many dealing with footprints/fingerprints left b different sources.

These fingerprints make it easy to find links.

The other side of the coin is, if it's easy for us to find links using the fingerprint, it's just as easy for the SEs to also ID these links, using the same fingerprints.