DH and I went on a tour recently, and the form that we have to sign up on requires you to initial that you haven't toured with them in the last 6 months, that you haven't done xyz, etc. That you make over xxx minimum dollar amount AND that if you are married that spouses are touring together.
When I thought that we were purchasing, we were given certain benefits if we "referred" our friends. I said that we have a number of friends that worked opposite hours, and wondered they could still do the tour. They told us no.
However, that being said, I don't believe they pre-screen or require any sort of proof, other than a credit card and license when you tour.