sijugk - 11:38 am on Mar 16, 2011 (gmt 0)
It seems Google detects a paid link by checking the link with the theme of the website. If the specific link is not adding any value to the user, Google may treat it as paid link (atleast a link intended to manipulate PR).
One motor car site linking to a cosmetic or film review site
This link is not providing any value to users and Google has all the right to decide it as a paid link (unless a nofollow tag is added). So the factor which determine whether a link is paid or natural is the relevancy of the link.