While any link is a good link, not all links are created equal. Here are a few of the key factors that go into deciding how healthy a site is, and in turn, how powerful a link from them will be for your SEO needs.

What is the PageRank?

What's the PageRank of the page that is linking to you? What are the PageRanks of some of its subpages that link to other sites? Install the Google Toolbar to find out. While some websites may have very high PageRanks for their homepage, the other pages on their site may have a much lower PageRank. Pay attention to the specific page that will be linking to you when considering how good a site's PageRank actually is.

Cache Status

Does the site have an up-to-date entry inside of Google's cache? You can check by searching Google with the command "cache:example.com". A site that doesn't have an up-to-date cache is often a red flag that there is something wrong with a search engine's ability to crawl that site.

Link Popularity

How many entries does the site have? What is its link popularity? Use Marketleap's Link Popularity Checker to get some quick and dirty facts about a site's web presence.

Number of Outbound Links

Does their links page already have a ton of links on it? The more links they have, the less link love your site will receive. Also, are there a lot of sponsored links to unrelated sites like sites about gambling, pharmacies, or adult content? These can also devalue a site's ability to pass on link value.

Age of Site

How long has a site been registered for? Usually, it's best to get links from sites that have been around for more than two years, as search engines tend to be a little less trusting of brand new domains. You can check how long a website has been around by going to the WaybackMachine and seeing when the earliest entry for the domain was.