Hi, and thanks for your medical question on eHealth forum!
There is no way by external appearance that you can know if a girl is a virgin or not. Usually they have an intact membrane (hymen) inside their vagina, which is slightly difficult to penetrate and painful for the girl in the beginning. This results in spotting or a slight bleed after the first intercourse. The hymen may break by fingering however this cannot be taken as a sign of losing virginity. Loss of virginity means first proper intercourse.
However in many girls, this may not be there if they have been very athletic in the past. In these girls the hymen could have ruptured due to athletic activities. Some girls have a very lax hymen. At times a fall resulting in high impact or a stretch of legs can rupture the hymen or make it very lax. Horse riding and cycling are two activities that can rupture the hymen or make it very lax. At times a girl is born with a torn hymen.
Usually if a girl is a virgin and even if the hymen is not intact, the vagina is tight and penetration not easy. Hence an intact hymen is not a sign of virginity and this is common knowledge now. Hope this helps. Take care!