Look, if you pass a girl in the hallway and she looks at you, then it could be just because you are in her line of vision. However, if a particular girl always seems to catch your eye, then the likelyhood is she is interested in you. The clue is to see how her eyes look, when they look at you.
There is no doubt in my mind that eyes give a person away, because unlike other body language, you can't really hide what your eyes are saying, even if you try to.
If someone gives you an evil look, but smiles, you know the smile is not real...
It depends. If it's just a flutter and she seems to be looking around at everything it's most likely nothingg. If she holds your eyes for a bit longer than normal she was totes checking you out.
depends on how long. if its a holding eye contact and not just a quick glance, she checking you out. if a girl holds eye contact and smiles for a couple seconds she's interested. it's called the 5 second flirt. that's when a girl holds eye contact with a guy for 5 seconds, while smiling, then looks away
Look in their right eye so it gets processed by the left part of her brain, and look at the direction of the break. Down (submissive) is good, sideways (neutral), up (dismissive) is bad.
Even if you have known her for years, keep looking to see how she breaks eye contact. Do not just use it as a initial indicator (It is/almost ALWAYS up, even when she was told/seen something she used to like from her guy and would break contact downward? Guess what is going to happen soon. Watch your friends girl with your friend too...).
The longer they hold the better. Most shy girls are going to look away quickly.
I never break eye contact first, and I try to be looking her in the eye before she even makes eye contact with me.