I did some analysis before on what is "natural links" -- took some sites like whitehouse.gov or dell.com which really get natural links and presumably have no paid links. if you analyze the anchor text they have coming it it is amazingly random --
Look at this bunch for dell.com:
Dell
www.dell.com
http://www.dell.com/
Dell Computer Corporation
Dell Computers
http://www.dell.com
Computers
Dell Technology
dell.com
Michael Dell's little dorm room enterprise
OptiPlex 745
Regular-sized Text
And of course there is a natural distribution to the frequency.
Hence it is REALLY easy to spot non-natural linking because the frequency distribution is entirely off.
Therefore now I make a big effort to throw in lots of variations in anchor text, and including some "click here" and stuff like that.