No it is impossible to "tell" if a girl is a virgin or not just by "looking" down there. Some doctors, especially in the Middle East, where virginity is a must for marriage, will say that if the hymen is torn or gone, that a girl is no longer a virgin. This is NOT necessarily the case, as the hymen (which is a small piece of tissue that covers the opening of the vagina)can be torn just by exercise, riding horses, doing the splits, tampons, etc. The only way any doctor could say you are definitly NOT a virgin is if you have had a child, which they can tell by the way the cervix looks.