you can´t. for guys physical attraction and the desire to have sex are indistinguishable from "really loving" a girl sometimes. just have sex if you want or don´t if you don´t want. making it depend on him wanting more thatn that or not in the long run will bring you no benefit. there´s a good chance, that he´s not gonna be the one you marry anyway.
If he is really trying to get to know you. At your age you can't have sex anyways so if you like a guy and he likes you but your not sure if he is just trying to get into your pants mention that you have to wait at least until your 18. If he sticks around then he wants to know/date you for you. If he leaves then he just wanted into your pants.
Sorry hun, You can't... Guys don't "like" girls except for their genitals. We tolerate you as long as it takes to get that part but generally women are not tolerable. You create rules we can not live by and punish us mentally and physically when we don't adhere to your standards of how we should act. You give us shit tests, you are over protective and jealous, we can't be ourselves around you, we are expected to stop being males and start thinking the way females do, which is impossible but you expect it anyways and then shame us when we are unable to live up to it.