In Rocky Horror Picture Show parlance, a "virgin" is anyone who's never seen RHPS (that's "Rocky Horror Picture Show," the movie) in a theater. Seeing it on TV doesn't count (TV virgin), renting the DVD doesn't count (Video virgin), and even seeing the play really doesn't count (Stage virgin). Personally, I have my doubts about the last one.
One of the easiest ways to tell an RHPS virgin if they arrive to the show prop-less. Of course, you can always come without props—they aren't necessary to have fun—but having the right props and using them at the right time can be a blast. Listed below is the "official" prop list, according to the Web site http://www.uta.fi/~cstivi/rocky.html