Judging the value of a potential link will help you to assess whether it is worth acquiring, and will give you an idea about the resources, and efforts you should be putting into it. One of the best ways to do that is to check the various metrics associated with the link. Following are the metrics.
Strength of the domain and Strength of the page
Domain strength is the combined value of an entire domain. Looking at the domain as a whole instead of looking at the individual pages will help you to understand how strong it is. Domain strength is commonly displayed as the Page Authority on the homepage or as Domain Authority. Google calculates PageRank on the basis of the number and the quality of the links pointing to a website.
PageRank of a website will give you an idea how strong the site is. Even though this is only the PageRank of a single page, it is still a good indicator of the strength of a domain, because the majority of a site's links will be to the homepage.
Site strength is a good metric to use. It will ensure that you get links from strong websites. This will add more strength to your own website. Google will take it as a good sign, and will provide you with a good ranking.
Following are some tools to measure the domain strength and the page strength.
PageRank extension (Chrome)
Google Toolbar for PageRank (Internet Explorer)
Anchor text
Anchor text can provide Google with an indication of the matter of the subject, of the page being linked to. For example, if you link to a page using the words 'healthy foods', then it is likely that the page being linked to will contain information regarding foods that are healthy. This information will then be used by Google as part of its ranking algorithms. In this case, Google may decide that the page being linked to should be ranked higher for the keyword 'healthy foods', and close variations.
It is better not to build a lot of links that have the exact same anchor in them, especially if the links are not high-quality links, like links from low-quality domains or links that have too much anchor text or non-editorial sitewide links. Try to make your link profile look as natural as possible. It basically means getting links that use your company name as the anchor text.
The following are some tools to measure anchor text:
Open Site Explorer (Moz.com)
AHREFs
Majestic SEO
Number of links
Always keep a track of the number of links you have built. You have to keep in mind that quality of the links is equally important as of the number of links. The number of links can be useful in two ways.
To measure the progress and success of a link building campaign
To compare your website with a competitor's website.
You have to factor in the quality of the links as well, for the above two uses to provide an accurate result. When you compare your number with that of your competitors, it can sometimes show gaps that may explain the difference in the ranking. For example, if you are trying to rank for the keyword 'smartphones', and the site on the first page of results all have over a thousand linking domains, then that gives you a good sense regarding the competitiveness of that keyword. This will give you an idea how much attention you will have to give in order to rank among those results.
The same tools listed above (for Anchor text) can be used to measure the number of links as well.
Linking root domains
It is a powerful ranking signal to Google. Linking root domains mean the number of distinct domains that link to your website. For example, if Fox News links to your website from four different news stories, then it will be counted as four links and one root domain because all the four links come from the single root domain Fox News. The number of linking root domains actually shows the true popularity of a website. Consider that Google thinks of links as votes. Then, no matter how many links you get from a particular website, Google will only count it as one vote since all the links come from one root domain.
Multiple links from the same domain can happen. One way is to link from a multiple content page. The other is sitewide links. A sitewide link is a link that is placed in a templated element of the website, like the sidebar or the header or the footer. However, they are not as valuable as in-content links from just a few pages.
The same tools listed above (for Anchor text) can be used to measure the linking root domains as well.
Suppose you get a link from the homepage of CNN to your website, which is all about chocolate.
Would you reject it because the CNN website is a news website, and not about chocolate? If the CNN website helps to bring traffic to your website, then why reject it? The point is, you can try to convert those visitors into potential customers. For this reason alone, you should be trying to place links on websites where potential customers may visit. While, whether Google takes page relevance as a metric to calculate the value of a link, may still be a debate, page relevance can be a good SEO strategy, and a potential direct income source.
Position of links on the web page
Google is able to work out the position of a link on a page. Based on the position, Google will value it.
For instance, if the link is at the bottom of the page (maybe in the footer of the page), Google may give it a less value, assuming that the link may not be very useful for the users since it is placed at the bottom of the page. Many users may not scroll down a page till the footer, and even if they do, they usually do not expect to find useful links in that section.
Understanding these metrics will also benefit you when you are doing a link profile analysis on your website or that of your competitor.