Who knows, but I bet it would be really easy to look at niches that have an exuberant amount of targeted anchor text, or potentially looking at ecommerce sites that have really, really high internal page Page Rank.

Or, simply look at all the rat reports (spam reports) and go from there.

Begin speculating what their methods could be

Maybe start at the Digital Point link exchange forums and follow signature links back to sites, and check out reciprocal linking behaviors – who’s participating in exchanges with them, etc?