Men of Reddit: How do you know when you want someone to be your gf rather than a "hook-up buddy"?

This is a question that comes up A LOT when I'm out with my girlfriends, so please Reddit, help some sisters out. :)

My question is: How do you make the distinction between a girl you want to be serious with and one you want to be "hook-up buddies" with? Is it an instant decision? And, how do you know when someone is girlfriend material?