Most Helpful Guy
A guy who is interested in you romantically (and not just sexually) will want to talk about your life, your interests, your future, etc. He's trying to find out if you have compatibility with him for a long-term relationship.
A guy who just wants sex will always steer the conversation towards sex, and will get bored quickly with other topics (because he's usually not interested).
It's usually not that hard to figure out which group a guy is in - other guys can usually figure it out pretty quickly.