If you're looking to analyze the legitimacy of a Twitter user it would probably be from the content they tweet. Creating unique tweets is really difficult at scale, so most just retweet or pull from some data source. And if they create enough unique original, valuable content, well https://xkcd.com/810/. Watching people argue with Markov chain generators, incidentally, is one of my favorite things in the world.
There are quite a few services that attempt to determine what percentage of an account's followers are legitimate, but I'm not sure how they do it; probably each a different way.
As an aside, I did watch someone sell a famous author 100,000 twitter followers for $15,000, only to turn around and buy those followers for $100 and take his family to Disneyland.