If a boy has done sex before how the girl will come to know?

One sign but not the most accurate sign is if the Hymen (Thin membrane tissue covering the entrance to the vagina) is torn or missing. But this could happen from a Fall, Masturbation, or previous sex.

The only tale tale way is to be honest and try to believe them. If you see them hanging around alot of men this does not mean they are having sex and therefore not a virgin.

Masturbation does not make you a nonvirgin. You only lose your virginity during Sexual Intercourse, (Penis to Vagina, Penis to Anal). Many consider oral sex to be one base away from home. So therefore many do not consider it sexual intercourse.