The twitteraudit service says they determine if an account is fake or real as:
“Each audit takes a sample of up to 5000 (or more, if you subscribe to Pro) Twitter followers for a user and calculates a score for each follower. This score is based on number of tweets, date of the last tweet, and ratio of followers to friends. We use these scores to determine whether any given user is real or fake. Of course, this scoring method is not perfect but it is a good way to tell if someone with lots of followers is likely to have increased their follower count by inorganic, fraudulent, or dishonest means.”
It seems that the service would determine a lot of plain lurker accounts as fake. (And I’m a lurker on Twitter who almost never tweets and has a really low followed/follower ratio, so I would probably get flagged as a bot. Actually I think Twitter flagged me as one before, because my account got suspended once even when I didn’t really post anything at all, and I heard someone being unable to see one of my comments) Most bots have probably evolved a lot from simply lurking and following, so I don’t think the service provides an accurate picture of bots on Twitter.