1. A quick glance
2. a hasty preparation
He gave my work the once over and left.
to casually turn your head towards a person of interest, then - with eyebrows raised - quickly and surreptiously (or blatantly, if you want to obvious about it) flick your eyes from their head down to their feet.
basically, checking someone out whether you find them attractive or not.
John gave the girl entering the party the once over.
(n) a sweeping glance down and up the body of an attractive person
(v) "to give one the once-over." to check out someone you find attractive by quickly looking them down and up, usually from a distance, head to toe. can be done discreetly or more obviously, by smiling or raising the eyebrows upon giving the subject of one's interest the once-over.
"just give him the old once-over, he'll know you're interested."
"i saw you giving her the once-over. don't you think she's cute?"
Brief glance in their general direction, be it due to a distraction they caused (that you don't find important), they're just somebody in the way of what you're searching, or they irritate you and you flash them a look of disgust.
Doing a once over on a criminal is not enough to get a detailed description to a police detective.
You're in a crowded hallway. You're looking for your friend. You look to your left, than to your right. There's no sign of him/her. That's a once-over.
"So, did you get a good look at the criminal?"
"Well, not really. I mean... he was ... male. And... erm... flat chested."
"That's the best you can do? Reports of other victims say you glanced in the his direction before he had his mask on."
"I just did a once over, man!"
"... A what?"
So now I put it on UD. Now I can tell the next person who asks, to look it up.