(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.
"i saw you giving her the once-over. don't you think she's cute?"
basically, checking someone out whether you find them attractive or not.
Doing a once over on a criminal is not enough to get a detailed description to a police detective.
"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.