Steve Rubel points to a great post by Robin Good. Robin offers ways to determine how much of an influencer a blogger, or anyone else online for that matter, is. Robins list suggests that you check sources such as Technorati and Alexa.
I would add a few ideas to the list:
In addition to Alexa, see who else links to them through Google or another search engine.
Check their advertising potential. If the blog has ads then they may have sizable traffic.
Check out their valuation on BlogShares.