It comes from some level of intuitive guess work, but it also from noticing people's behaviors and being inquisitive about feelings as a knee-jerk reaction to everything. It sounds scary that there are people who feel like they know you before you think that they have any idea about your personality, but I can say for myself I can just read people and guess. I have to get to know them at some sort of level, of course, but it's to the point where my close friends don't really second-guess my observations. I want to reply later with a more thought out answer, but I have to say that while it may seem out there, a high level of empathy might not be over-estimated.