Can you usually tell if the other person wants to be your girlfriend before you have with them?

how can you tell if a girl wants to be in a relationship or just a one night thing before you have with them. so many girls after hanging out with them after one or two weekends bring me back to their room. and so many times it kind feels to promiscuous so i do not sleep with them but then they loose contact with me. what can i do to tell if they want to just sleep with me or actually have a relationship with me. what can i do to let them know i dont want random i only want to be with someone who i am actually in love with. I know in college many people say just experiment and dont be so up tight but i am a virgin and i think it is legitimate to want to only loose it with someone who actually cares about you.