Actually, if you are paying attention you pretty much can tell if a girl is a virgin... depending. But it's not something physical, it's more behavioural. There is a sense of a certain "unknowingness" that can come up in many different forms, kockiness, naivete, over zelousness. It can look like a lot of things but basically it is about thinking that sex is something that it is not... since without having experienced it you can't actually know what it in fact is. Odds are the average man would have NO idea however, and if he were a virgin himself wouldn't even know to look.