The
TwitterAudit.com is a good tool to detect fake or farm followers. It can be used as one of tool to detect scammers on Twitter. If followers are mainly fake it is not good.
You can randomly click on some followers to check their account ages and audit scores for those accounts too.
I am not sure scammers are readily to spend funds to buy followers (I doubt that they won't) but there is a site for
Buying Twitter Followers with Twiends.com and you should know about it. It is easy to naturally increase your followers on Twitter so personally I don't use that service.