All you have to do is visit http://fakers.statuspeople.com/, connect to Twitter, and authorize the app. Your score will be displayed in percentage and divided into three categories: fake, inactive, and good.
We take a sample of your follower data. Up to 1,000 records depending on how ‘popular’ you are and assess them against a number of simple spam criteria.
On a very basic level spam accounts tend to have few or no followers and few or no tweets. But in contrast they tend to follow a lot of other accounts.
(Source: About page)
You can use Fake Follower Check to check other Twitter profiles.
You can also get your own dashboard for a small monthly fee. You will see who your fake followers are, decide if you want to block them, and have the ability track up to five accounts.