People used to believe you could tell if a girl was a virgin by looking to see if her hymen was torn. The hymen is a thin membrane that stretches partway over the opening to the vagina. When a girl has intercourse for the first time, this membrane usually tears, causing some pain and light bleeding.
In many cultures, bleeding at first intercourse is still considered solid proof that a girl is a virgin. But, we know a hymen can be torn as a woman's body grows and develops. Bike-riding, using Tampons, or doing certain sports, like gymnastics, can also cause the hymen to tear. Some girls are born without a hymen. And, it's real common for some girls not to bleed at all the first time they have sexual intercourse.
It's impossible for a doctor to examine a girl and tell if she is a virgin. If she still as an intact hymen, then she probably has not had vaginal intercourse. But, if she has no hymen, there could be many reasons for this, and only her word is the definitive answer.
It is also impossible to tell if a guy is a virgin. Even a doctor can't tell. Similar to the situation for women above, only his word is the definitive answer. Taken from: www.sexetc.org