Link building is one of the most difficult activities to measure. Search engines internally weigh each link with precise, mathematical metrics. These metrics include the following:

Ranking for Relevant Search Terms
One of the best ways to determine how well a search engine values a given page is to search for some of the keywords and phrases that page targets (particularly those in the title tag and headline). For example, if you are trying to rank for the phrase “music video”, earning links from pages that already rank for this phrase would help significantly.

Competitor’s Backlinks
By examining the backlinks of a website that already ranks well for your targeted keyword phrase, you gain valuable intelligence about the links that help them achieve this ranking. Using backlinks tools can discover these links and target these domains in their own link building campaigns.

Number Of Links On A Page
It appears to be important or at least certainly something to be aware of getting linked-to by a page with few links is better than being linked-to by the same page with many links on it.

Potential Referral Traffic
Link building has many advantages, links not only tend to provide better search engine value for rankings, but send high amounts of direct click-through traffic to your site.