The latter, duh. A simple glance does not mean, "I need you to fertilize my womb right now." A girl could be looking for any number of reasons. Maybe she finds you attractive. Maybe you're doing something hilarious or stupid. Maybe her friend finds you attractive and she's trying to figure out why. Maybe you have something stuck to your ass. It can be an indicator of interest, but of course it is not always.
If you catch them looking more than once, then usually they are checking you out. Of course, this isn't ALWAYS true but usually it is. So the next time you catch her looking, smile and if she smiles back then go up and strike a conversation.
or maybe you have this one bugger hanging from your left nostril.
If he looks good to me
Okay, so to answer the question about a girl checking out a guy. When I'm checking out a guy I will look at him for a very long time. It's not a passing glance. Most passing glaces are looking to see what another person's position is so I don't bump into them. Just like I look at the furniture.