HELP! How do I know if he really is single. All my co-workers tell me he isn't but I've fallen for him?

This guy I have been seeing that I work with has been totally trying to be with me, however I've been hearing from other co-workers that he has a girlfriend/fiance? I asked him personally if this info was true and he completely denies all allegations. Im really falling for him and don't know what to do? I really need help?