1. Quality of domain name: Usually the quality of the domain name is a great indicator for how spammy the site is, or if it is simply being made to sell links. Try and stick to domains that actually look decent, rather than mambo jambo.
2. How many links have been bought already: If a webmaster sells a lot of links on a web page, the link equity derived from outbound link is reduced. One can’t do much about links sold in the future, but it’s best to target pages that haven’t already sold many links. Also look at the quality of the other links that have been bought since it’s best to avoid bad neighbourhoods.
3. Placement of your link: Higher up the page will derive greater link equity. If you can get the first link on the page then you’ll gain the largest amount of link equity you can.
4. Has the site been penalised by Google: Google obviously does not like individuals buying links, so often you will find that pages selling them will be penalised. Having links on such pages can actually harm your rankings, so avoid like the plague. One indicator that a page has been filtered is that the PR is significantly lower than the SEOmoz rank.
5. Is the PR Valid: Someone might be selling you a PR5 link but they may be faking the PR. It’s quite hard to spot someone when they do this to increase it fractionally, but occasionally you’ll come across people that will try and pass of a PR6 domain that only has a few back links. Most days I’ll come across this. I tend to trust SEOmoz rank rather than Google PR despite the fact SEOmoz rank is only a guess itself.