How do you know that a girl is not virgin?

You see if she can tame and ride a unicorn. . .

There is no accurate way for a regular person to determine if someone is a virgin. While a gynecologist can determine if a female does have an intact hymen, some girls are born without a hymen; many girls rupture their hymen through sports or masturbation. Not all women bleed or experience discomfort with first time vaginal intercourse.