You are probably better at detecting bots than you think. The same way you can recognise phishing and spam due to spelling or syntax errors. If I get a new follower and the only contact they've made with me is via Twitter, and their tweets are totally odd or cryptic, they aren't adding to my experience so away they go.

I've only had one false positive I know of and it was one of my wife's friends. She had a profile picture of her teenage daughters and no tweet history. I thought 'what the hell is a teenage girl doing following me - guaranteed spammer' only to find out who it was the next day. I unblocked and no offense was taken and no harm done.

Still, I guess I'm a bit stricter than average and this is something that most people don't care about.