An unique project in forum building

I am working on sort of a secret project at Night Shaded but here's the gist of it. We want to market our community to a specific group of people. Basically, to increase the quality of the community, we are going to restrict registration to require that you can prove that you fit with this group of people. Our staff manually reviews the applications and what not to make sure that this person is the real deal and should be allowed in. Now what would be the best way to go about doing that?

All depends on the market. If this was a police officer community, you could ask the member to provide his department and badge number, meaning there is something tabgible to prove. However, if this was going to be a community for only people who like green ballons, well how do they prove that?