How to know if a girl is interested

[Little delights]

He girls, I believe I may have a problem. I can never tell if a girl is truly interested me at first glances. Sure she may smile back but how do I know? Give me some feedback on how to know what to look for?

[Yet another guy]

First test is to ask her out on a date. If she says 'yes', she might be interested.
Once you've gone on dates and had sex a few times, it's likely that she's interested.

[NeverDated]

At first glance? Hrm...

If you catch her looking at you, and she looks away with a little smile, she's interested. Caveat: If she looks to something else purposefully (a friend, a sign, a cell phone), she's just being polite with her smile. Approach if you're really interested, since she's noticed you.

If she maintains eye contact casually, she's interested. Caveat: If she looks very intense/serious, she's trying to get you to stop looking at her. Don't approach.

If you catch her looking at you repeatedly, she's interested. Caveat: Make sure you don't have something on your face/shirt, or are dressed like an idiot. Then she's just gawking. Approach if you're really interested, as you've already made yourself look silly and she may appreciate the bravery.

If she starts playing with her hair/necklace, rubbing her neck/shoulders, she's feeling self-conscious (because she's interested and you're looking at her). Caveat: If she plays with her earrings, watch or touches her lower arms, she's uncomfortable, so probably not interested.