How do you prove you're a virgin in the town that inspired "Sex and the City''? The producers of an off-Broadway show are giving away free tickets to anyone who can demonstrate his or her chastity. Which raises the question: Just how will the theater know? Producer Ken Davenport, 34, had a hypnotist screen people standing in line for the free tickets to the first preview of his 90-minute comedy, "My First Time'' to determine their status. The show, which opens July 28, is based on a decade-old website that invites people to anonymously share their stories about losing their virginity. Great, another "who's done it" play - just what Broadway needed!