The hymen, a thin fold of mucus membrane covering the opening of the vagina, was once considered proof of virginity. However, the fact that the hymen can be broken and/or stretched by other things, such as bike riding, volleyball, etc, means that a woman whose hymen isn't intact isn't neccessarily a non-virgin. And some woman are simply born without them.
However, if you found an intact hymen on a woman, then it would be definite proof of virginity.
This are what hymens can look like:
