I am working with someone who has numerous inbound links. Some are a result of direct contact, with no payments involved, and some are paid for links. In general, although it fluctuates, the paid for links probably have the higher ranking themselves. The page rank of our site has improved a lot. My question is whether or not the links have an indefinite ongoing value. How can I distinguish whether or not continual payment to "paid for" links is necessary or not? Is testing the waters by simply dropping those links the only way to know?