I doubt you're going to love the answer but it's basically "you just know."
It super depends on the person, the moment and the vibe. If it's been a date or two, maybe we'll be talking, turn face to face, the conversation fades, we're looking at each other and we both know we're gonna kiss, so we do. If we're all dancing, maybe you lock eyes and kind of slide into each other. Maybe you're at a party and jokes lead to childish bumping with your shoulder on the couch or whatever and you see how they respond.
If I had to boil it down for non drunk dancy events (those are just raw physical chemistry) as simple as I can it'd be something like (also, I'm just assuming genders etc, adjust as necessary):
Is she laughing at your jokes? Is she actively engaging in the conversation or just being polite while you talk at her?
Sounds dumb but is she playing with her hair? Looking into your eyes? Leaning in?
If this is all fairly new to you, then I'd play it safe and at some point in the conversation find something you both enjoy and could do together (maybe you like the same terrible dive bar, maybe there's a nice walk nearby, maybe somewhere fun for tapas) and then later on, when you think things have clicked, say something like "it's been amazing meeting you, but I really shouldn't abandon my friend Bill, even if you are way funnier and probably better looking. But maybe next week, can I buy you a drink at that dive bar/walk/tapas?"
Be prepared for rejection. It happens. It's not the end of the world, though you might feel it is at the time.