You can generally tell if your follower is real by looking at their twitter stream. If all they tweet is “omg porn” or “I have hot pix”, they are spam. And you can follow the Twitter spam folks (I’ll grab their name in a moment), they’ll follow you back, and you can DM the suspected spammer’s name to them and they’ll look into it.
They could be a commercial user (an eatery or, I don’t know, t-shirt store) that promotes on Twitter and for some reason things you’d be interested and that is why they followed you. Or is interested in you. On Twitter I’d say block ‘em if you don’t like ‘em, but they could be legit users.
Finally if they have full, well used twitter streams that aren’t all “see my porn pix” or “buy viagra” or whatever and details someone’s life or interactions, they’re probably a real person.
As an example for the middle one, a local eatery follows my foodie twitter stream because I tweet about food. Yes, theirs is used professionally, yes they only tweet about their food, promotions, stuff like that, but it is appropriate for them to follow a foodie tweet, as they are a foodie place. You know? They aren’t trying to spam.
At the same time, there are people who fall below one and two who might be trying to promote their stuff using Twitter appropriately, but haven’t quite figured out what that means.