Using the follows of the paid users. So unless you subscribe to spam you shouldn’t get it.
Some users follow a _lot_ of others, though, and might have a higher risk of following spam/impersonation accounts. As a refinement, maybe you could score each follow by dividing by the paid user's total follow count, and only include users with a follow score above a certain level, e.g. 1/200. And/or disregard the follows of users who follow identified spammers/impersonators.