I tend to do this a lot when it comes to girls and its something that I feel makes me retarded, but how do you tell the difference between when a girl is just being nice to you and when a girl is flirting/sexually attracted to you.

For example, today I went to this even they were holding for college and a girl was near me and she turned to me and she started talking to me. Now, I assume that she was just being nice and social, but how do I know. She asked where I was from and I told her and then she told me where she was from and how her boyfriend was living near where I was living. I wasn't planning on trying anything with her, she was pretty overweight and not that attractive, not that I'm any better.