Originally Posted by AVC

Google is smarter than you think, they have thought out all scenarios here, they see patterns in linking via sophisticated link mapping technologies that webmasters do not have, they can catch all reciprocal and triangular linking schemes easy, that is their business and they score sites depending on how many rules a webmaster breaks.

They know that in many cases webmasters reciprocate back to directories they used automated submission services with and this is how they can determine you purchased the service from a link farm or link broker or an "SEO professional".