Link builders are used to looking for something tangible to determine if a link is worth acquiring or not. The most tangible metric they have had at their disposal over the last several years is Google’s PageRank. This was the measurement that Google has been using to report the importance of a specific webpage. The biggest problem is that it isn’t extremely valuable when building links other than that it can be used as a directional tool to determine if a page is popular or not.
So when you are building links what should you look for? The most important thing to consider is if a website is relevant to your website. Google is pretty good about determining the themes of the billions of webpages out there. They know that search engine optimization is a business and they try to prevent cheating as much as possible. Links from unrelated pages often carry less value in the big picture because of this. Google is looking to see if people in your own industry see you as an authority. This is much more important to their search algorithm than a link from a random website that has high page rank. So in theory, but most likely reality, a link from a PR 3 page that is related to your website is much more valuable than a PR 7 link from an unrelated site.
Another metric you should look at is the number and quality of the links that point to the page of which you are looking to acquire the link on. If they are getting links from spammy sources, then the page probably won’t pass much power back to yours. If they aren’t getting quality links for their website, you probably won’t be receiving a quality link in terms of power back from them.