Used to describe the action of checking a girl out with the intention of being inconspicuous but being completely obvious. Derived from the slang term, casing the joint.
Jimmy was casing the Korean girl walking across the street using his sunglasses.
Kanye was casing the booty all night long.
Or casing sausage, is when you get out of the pool and your shorts are sticking to your legs so tight that it looks like they vacuumed sealed your penis, and it becomes a very visible outline.
Girl at the pool: Hey, do you see that guy over there?
Friend: Yeah, why?
Girl at the pool: Look at his shorts, he is totally casing.
Friend: Yeah, look at it, its pretty big.