People very often worry about their followers on Twitter. There are good reasons for this of course, but it's one of those situations that I really think just isn't worth losing sleep over. Then of course you get the very sad people who will pay actual real dollars for 'followers' or who will even buy an existing Twitter account and its followers. In all honesty, I pay very little attention to the number of followers that someone has got, since I'm more interested in what they say, how they say it and how often they say it. If I'm getting good content from them, they could be followed by all sorts of unsavoury characters for all I care.
However, for those people who do worry about such things, there is the Status People Fake Follower Check which is very simple to use. Simple provide it with your Twitter name, allow it access to the account, and it will check up to 500 of your followers. You then get a total return based on 3 variables, Fake, Inactive and Good. Mine came out at 5% Fake, 18% Inactive and 77% good. I'm pretty happy with that result - I certainly know who a lot of the fake accounts are, which is one reason why I haven't followed them back. A fair number of accounts will be inactive, at least partly because people signed up to Twitter on one of my courses and were then unable to use it at work.
Once you've got your own score, you can then start digging through your friends scores as well. I'm not entirely sure that's a good thing, and it's a bit like poking around their house when they're not at home. You can also do anyone else that you like as well - I tried @ladygaga and she has figures of 34% Fake, 38% Inactive and only 28% Good. Mind you, that still adds up to a lot more than any I'll ever have!
Anywho, give it a go. It only takes a few seconds, doesn't tweet results without permission and it's quite interesting - certainly on corporate accounts.