How do I better perceive when a girl is attracted to me?

Hello everyone.

So I'm 23 years old and for lack of a better word I'm completely oblivious to girls that are attracted to me. I've always had this problem and I think that in the past it has caused me to miss out on what could have really been some great relationships.

An example of this happened today. One of my friends ''Whom is Female'' needed to go to Target to get a few things and she asked if I would tag along with her. Well when we were checking out and getting ready to leave, I had picked up a magazine off of the end cap and was thumbing threw it. I had noticed the very cute Asian girl that was working the cash register and her and I even shared a nice smile with each other...Then I got lost thumbing threw the magazine. When we got into my pickup outside my friend looked at me and asked what the hell was wrong with me? I had no idea what I had done, She told me that the girl at the register was really into me and she could tell that she was just waiting for me talk to her. I of course had no idea of this.

But that's just one example...This happened many times in the past...A girl will be kinda into me and leaving subtle hints that she likes me...And I never pick up on these. I guess I missed out on that class in man training ''Subtle hints 101''.

If any of you girls out there can give me some tips it'll be much appreciated.

Thanks!

Nathan.