Look over to her, and hold eye contact for about two seconds. If she catches you looking, just hold her gaze for two seconds, smile, and say hi as she gets close to you. Don't say hi to her if she's too far away. If she's too far for you to say hi, then just hold eye contact with her for the same two seconds, then look away. Or even better, look down slowly, seductively. This is very flirtatious.
Either way, she KNOWS you’re interested, and you want her to know without you having to approach her and say, "I like you" (girls appreciate subtlety).
Be careful with the length of time: too short and she’ll think you’re afraid of her, too long and she might think that you think there’s something wrong with her. Or even worse, she might think there's something wrong with you.