Link building is one of the essential tactics all SEOs must know how to do if they have any hopes of getting a website to rank in Google's organic results. This will remain the case until the day in which Google decides to move away from a links based ranking algorithm.
A big part of the link building process is determining what the value of a link actually is, because without that you'll have no clue as to how much time and effort to invest into getting it. So let's take a look at a few metrics that will help you make that judgment about a link.
Does it even pass value? There are many links out there than will simply not pass any link value. If you are looking for links that will help your rankings then this should be the first thing you check. Here is a quick rundown of what you should check for.
Check for nofollow. Use of nofollow will prevent any link value from being passed. You will need to check the Meta Robots tag, the X-Robots-Tag within the http headers and the actual link for nofollow.
Check for use of anything other than a regular html link. The link will most likely not be passing any value if it's using something other than html.
Watch out for redirects. If the links are being passed through a redirect then the majority of the time they are not going to count.
Cache date The more trust a document or domain has in Google the more often they will revisit it and update their cache of it. You can check the cache date in Google by searching cache:www.example.com/page-you-want-to-check/. Look for something within the last 3 or 4 weeks.
Quantity and quality of links to domain The amount of links a domain has pointing to it and the quality of those links are going to influence the amount of juice you are going to get from a link. To check the links for a domain, go to Yahoo and type in linkdomain:www.example.com -site:www.example.com. This will show you which links are pointing to the domain. Check both the amount (higher is better) of pages linking to it and the quality (you can use these 8 metrics) of those pages.
Quantity and quality of links to page Run through the same process as you did when checking the domain's links except change your query in Yahoo to link:www.example.com/page-you-want-to-check/ site:www.example.com.
Internal links pointing to the page A link from a page that is linked to from other pages on the same domain is going to pass more value on to your site than one that is not linked to internally. To check the amount of internal links pointing to the page go to Yahoo and type in link:www.example.com/page-you-want-to-check/ site:example.com. Look for pages that are heavily linked to from other important pages on the domain.
Location on page The sidebar and footer are a common location of bought links, which Google is working to devalue. Getting a link from one of those locations may not pass on as much link popularity as getting one from with the body copy.
Where does the page rank? Is the page you are looking to get a link from ranking in Google for your targeted term? I don't think it's necessary to go into why this is good, it should be obvious.
Does it work for others? Find other sites that are being linked to from the page in question then grab the anchor text and do a search for it in Google. Does it rank for that term? Now this will not give you an absolute answer, because there are many other factors that could influence the ranking of those pages, but if none of the pages that are linked to rank for the anchor text then something could be off.
There are hundreds of other factors that you could consider when determining link value, but those 8 should give you a good starting point for your next link building campaign.