SEO by the Sea has a great post about spam analysis done at MSN. While MSN's credibility on finding web spam might be a bit questionable (since they rank it so well), but the research ran through a variety of factors associated with web spam. Most were related to temporal page variance and running things like inlinks and outlinks through power laws. If you hate to read research pages Bill also mentioned this 36 minute video.

Power laws are probably not something that many low level search spammers look at, but if you are going to do well at it, then it helps to see where you fit in the big picture and how it affects you. Although I have noticed some sites (particularly old, well-established ones) that have done pretty stupid things that should flag penalties that do not. The biggest thing that makes spamming easier than it should be is the number of badly coded sites with good content, but if you have the same number of links on every page, the same page size on every page, and the same link profiles as many spam sites you can expect that your site stands a good chance to get penalized.