If it happens more than a few times then you can say she might be interested. But, just because she looked at you it doesn't mean she is interested. You have to have more signs of interest than just that, (Does she try to talk to you? Does she always seem to pop up in places you frequently visit? Has she ever touched you? etc.)
It's how she looks at you. If she just looks neutral, then it doesn't really mean anything. But if she looks at you like you're a double bacon cheeseburger after she just missed breakfast and lunch, then she likes you.
Its something called the sexy Eye . If she looks at you and you catch her and she just smiles and turns away, Then she is interested , but if she just turns away with not emotion at all ,there is nothing there
you can't really figure out a girl that way,only way is to talk to her,i mean I had girls stare at me and smile at me but at the end of day they did it because they found me attractive but not interested in going out with me