TwitterAudit gives a single score of the percentage of any user's followers that are real. Most people are above 40%, but some are below 10%, suggesting that they probably bought a lot of them.
Check it out! Any feedback would be great.
It would be nice if I could pay to run a few audits at once.
Edit: so I played around with it for a bit, a problem it seems to have is it doesn't cope with Twitter "lurkers", people that don't tweet much, follow lots of people and have a few followers. You shouldn't count those that follow ~200 people but don't tweet much as "fake". You should assume that every user with under 1-200 followings is legitimate, because it's not cost effective for the follower sellers to do it with accounts with so few follows. or at the very least have their "score" very low.
http://twitteraudit.com/notch http://twitteraudit.com/marc_irl http://twitteraudit.com/redstonewire
Their followers are a lot of Twitter "lurkers".
There are a LOT of follow robots out there. Fake followers that follow lots of people, hoping to get followed back. If you don't, they unfollow.
Feature request: show a list of the fake followers for one's account.