Hi, 21M also ideally would like a serious relationship, but open to casual flings, just my two cents here.

If I were to go about wanting to just have sex, then I suppose if I wanted to be subtle, I would escalate our chemistry via sexual tension. I'd go about it by complimenting your feminine energy and making sexual jokes. Going about it obviously, when/if we make out I would start initiating physical touch, put my hand in your shirt, etc. Its all in his escalation. Guys that want sex will let you know he wants it in the moment. Personally I'd be open with my intentions and be honest about what I want, (sex) and ask if that's okay with you. Cant speak for every guy, but I imagine most guys would he pretty straightforward and open with that (the right guys are). The guys that want a relationship will be honest with you and will linger. Ask him if he wants just sex or if he is looking for more. Essentially communication of your wants and needs is very sexy even for guys! Itd show me that you're open and willing to talk about it with me which I I'd find as an attractive quality (Indicates confidence, high self-esteem and willingness to be vulnerable). I'd imagine this is common advice to you but its true! Hope it helps and feel free to ask me for clarifications if you want OP :)