How do you know the girl likes sex?

Well, depends what kinda girl you're talking about. If she's a girlie girl, she will always laugh when she's with you, which kind of looks fake, but she really means it. For that girl who acts cool all the time, she'll become nervous and less-cool around you. Give hints that you like her. Usually when a girl hears someone likes her, she'll be like "oh, he's so sweet" and then she'll like him back. Girls usually never show their feelings clearly to guys, but believe me, inside they are thinking about you constantly! And if a girl ever turns you down, just be like "She's not worth it." because every guy deserves a sweet girl.