Not even an experienced doctor can tell for sure if a woman is a virgin by looking at her hymen or vagina. This animated video attempts to explain further why virginity or hymen doesn't matter while making love.
Research has shown that a lot of girls don’t bleed the first time. For one thing, your hymen might have been stretched by using tampons or doing sport like horse riding. You might have a very thin or stretchy hymen, or have been born without a hymen at all. What is important while having sex is to make woman feel relaxed. That will help her feel comfortable and also lubricate. Being nervous or tensed will only tighten the opening your vagina.
Read more myths about virginity here!