There is no way to tell whether a woman is a virgin. This is because virginity is a social construct that isn't rooted in biology; commonly touted physical "signs" of virginity, such as the state of the hymen, are unreliable.
According to Planned Parenthood, most people agree that a virgin is someone who has never had sex before, but very few people agree what sex constitutes in this context. Even if it is agreed that sex means vaginal intercourse, there is no way to tell for sure whether a woman is a virgin by examining her. An unstretched hymen is often touted as a sign of virginity, but the hymen can stretch when a woman masturbates, uses a tampon or even rides a bicycle. Even a doctor can only tell a woman has had sex before if there are signs of recent intercourse or symptoms of a sexually transmitted infection.