As you keep using Twitter and have more and more tweets to be found by users, you find that you amass followers at a faster rate. If you tend to "follow back", which many people do, particularly early on, this trend increases somewhat.
Needless to say, some of these new followers are spammers. Obviously they're not that hard to identify when you look at their profiles. But as your following increases more quickly, you have less time to do this. So here are few things to keep an eye out for to speed up the process:
Firstly, they're often female, and their profiles include photos of attractive young women. They often have strange names that seem kind of exotic. Clearly this is because they've been created by some kind of name generating software.
They often have very few tweets. If you do take the time to look at them you'll see that they just describe completely unrelated stuff that comes out of nowhere. (Hard to believe that they fool people enough to get clicks on their links. Still, they must. Otherwise the spammers wouldn't be doing this.)
They also tend to follow many more people than follow them back. (This tendency is not confined to spammers, though. There are a lot of genuine users who do this, many of whom are certainly worth following.)
The main characteristic seems to be that most don't even have descriptions included in their profiles. So they stand out immediately in the list of all the others who've recently followed you. (Again, some genuine users might not include a description. But they're very rare, and would probably add one eventually anyway.)
Also, these Twitter spammers usually roam in packs. You tend to get clusters of them appearing from time to time.