That's really the question, isn't it? How do they discount paid links without too much collateral damage? Really finding paid links is a subset of the problem of detecting "lack of editorial control."
I mentioned one way in a previous post. This is the bad neighborhood theory. If you link to known bad things, google ignores your links. A variation of this is that you can detect a link seller if he links to known link buyers. Having a database of known link buyers would help tune this kind of algorithm.
A Bad neighborhood might be a score, not just a yes/no penalty. Also, it is possible that "distrust" may pass back through more than one level of linking. Think of it as a "reverse page rank." If this were true (see two clicks from lesbian porn), you would be responsible not just for what you link to, but what the people you link to are linking to.
Another way to detect paid links is to look at phrase co-occurrance statistics. Boy, that "texas hold'em" link sure sticks out on your needlepoint site.
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