by his behavior if he move things between his girl and him fast you'll know that he just wants sex, or the guy flirts with you and say i dont wanna hurt you crap at the same time, its also a sign of that he just wants sex, or he will say something that will lead the relationship to friends with benefits.
If he is interested in getting to know you, it should be pretty apparent. He'll ask questions to learn more about you and be willing to spend the time to hold conversations with you. Make sure to show him the same behavior if the interest is mutual. If he is only interested in sex he might be a bit speedy when making out, quick to go under your shirt or take your hand and put it on his crotch. Now of course he could still be a nice guy and just be excited, so make it clear to him that you want to pursue something deeper by telling him exactly how far you'd like to go that night. Make sure not to waiver, the foundation to a good relationship takes patience
If I'm genuinely interested in someone: I'll make time for them I'll bring them around to hang out with my friends I'll want them to spend the night, not leave I'll make us breakfast (with scallops if I really like her a lot) I'll be making a lot of effort to get to know her and see how compatible we might be long term
If a guy is interested in you, he'll ask you out on dates often and will not have a issue becoming exclusive. My boyfriend asked me to be exclusive after 2 weeks of dating. That's how I knew. But it's always good to wait till a guy has actually asked you to be his girlfriend till you have sex. That's what I do and I've never regretted it.
He will not bring you around his friends, won't ask any questions to find out about you, and won't tell you anything about him, goes couple days with out texting you, you have to text him in order to talk, won't take you on a date and if he's gone for a month and comes back just wants sex, wants naked picks of you, wants you to come over late at night, and talks sexual with you, if you feel like thats all he wants is sex and is useing you always go with your insticts, ih and if they tell you they liove you and you know dam well he doesn't even know you and you'll haven't been talking long just wants to get in your pants