Going from the social marketing angle, rather than looking at it from the search engine’s point of view, I’d look for links from high trustrank sites – these would typically be authoritative blogs and websites in your niche, and the traffic will send over will be higher quality too.

One indispensable tool I have installed in my Firefox browser is the SEO Quake extension. (extension = Firefox plugin).

SEO Quake is available for both Firefox and Internet Explorer.

When I’m at a search engine result page, SEO Quake will gather various analytics data and domain data.

For example, when combining Google Blog Search with SEO Quake:

It’s a little small, but below each search result is the number of google links, yahoo links, domain age, alexa ranking, del.icio.us links and other information is shown.

[For the actual SERP page, click here]

I wouldn’t use a single metric as a guide, but taken together a high trust site would tend to be older and have multiple backlinks for the search engines.

If you’d like to audit a blog, here is the search string at Google BlogSearch:

And if you’d like to monitor specific blogs, you can configure the Google Blog Search alerts which function in a similar manner to Google News Alerts:

SEO Quake and Blog Search is a good combination for me to measure the quality of inbound links.

If you’d prefer an alternative to SEO Quake, I know Seer Interactive’s Wil Reynolds likes Aaron Wall’s SEO for Firefox extension.

You can find the link for SEO for Firefox.

For more SEO advice, you can also check out Aaron’s SEO Book.