How do you know a girls panties are wet?

The lubrication of the vagina is heralded by a slightly viscous, shiny liquid that will coat the interior of her labia minor (the flesh that covers her vagina) and vaginal walls, this liquid may also present itself on her external genitals or labia major. This liquid is present to facilitate the act of intercourse and is one indication of physical arousal in a girl. Other indications are an increased blood flow to the vulva and mons pubis (pubic mound, area of slightly fatty tissue above the vulva that becomes covered in hair during puberty), which results in the tissues of the vulva becoming red and the erection of the clitoris (which is the topic of an entirely different question altogether).