Jadarendir said:
Unfortunately, you can't. It's the second law of sexual dynamics: If you think a girl is attracted to you, you are wrong. [http://questionablecontent.net/view.php?comic=64]
Laughable advice. I always assume attraction until proven otherwise. If your looks are average, caeteris paribus, enter any room with a population of girls for the first time, and at least one will be attracted to you on the spot.
My best points of reference are eye contact, proximity, and willingness to follow my leads. I prefer to keep them simple because these are to me the least likely to lead to confusion.
A) Eye contact: On first eye contact she lowers her eyes, looks at me fleetingly more than once, or keeps a lingering contact; the longer the better. I don't seek them checking me out, because girls are VERY adapt at checking out a guy without being seen. If I see a woman immediately looking elsewhere when I turn to look at her, I know she was checking me out.
Maintaining first eye contact is extremely important. Flinch first and your chances are as good as gone.
B) Proximity: Girls who are attracted to you will not back up if you move inside her personal space. She'll fidget, play with her clothes or hair, when you look at her intently or tease her closer. She won't buckle, move backwards, or make funny faces when you touch her, and she will seek your presence close to her.
That is why touching her from the start is paramount, because it indicates to her that it should be normal for you to touch her, that you aren't shy about it, and she'll be more relaxed when you move closer to her or reach for her. And when reaching out the first time, even when high-fiving or shaking hands, I'd always keep the touch longer than expected, just a few seconds more lingering, before either pushing them slightly or saying "allright, that's enough for ya."
C) Following the lead: To me, the best indicator. If she complies with your instructions when you play with her, when you test her, when you tease her, chances are good that she is attracted to you. I try always to reward her each time she does comply to something, either by hugging her, tugging her, massaging her, and so on, and withhold when she doesn't. These tests are great to allow some plausible deniability to touch her more intently.
If I'm with a girl in a bar or a social gathering and I tell, "I'm hungry, let's go grab a burger" and she follows me without resistance, I'm 99% sure she is attracted to me. No woman would follow a male stranger in isolation somewhere unknown without some level of comfort and trust, it implies that she trusts me to protect her. And guess what, these make excellent impromptu dates.