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.
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.
1. A quick glance
2. a hasty preparation
He gave my work the once over and left.