Most Helpful Girl
Funnily, this is a problem I feel a guy is having with me right now.
I think you know when you're around them you have thoughts that don't begin automatically with 'oh they're so hot', or thinking about doing physical things with them. I think you'd be able to tell when you can see personality traits that you are really into and hanging out is more about talking and having fun that way, without necessarily physical stuff.
Most Helpful Guy
I like them, then sex's coming..