“Hi. Not wanting to make a complete arse of myself I’m sure you don’t either and you are.”
First of all, it depends on whether she’s got a nice butt or not. If she doesn’t, tell her; if she does, just enjoy the situation.
Or you would request that someone sing Blue Moon and dedicate it to her.
Oh good heavens I’d just walk over and tell her her dress was caught in her drawers. And yes, I’d want to be told, and been thankful when I have been told that my zipper is down and my member is hanging out and in danger of being caught in a pinch-roller on the automated check-in machine. Happens more often than you’d think.
My favourite response to telling a guy that his fly is undone is still a neighbour from my childhood - “Don’t look then.”
I think I’d probably just quietly tell her myself, too. I was walking to work one morning, and I saw another woman across the street from me whose skirt was riding up under her knapsack so her whole butt was showing. She was too far away from me or I would have let her know why her butt felt so strangely cold.
I’d go with Arrendajo’s solution. She’s going to be embarassed when she finds out no matter what you do, so talking in riddles isn’t going to help. The sooner she realises what the problem is, the sooner the embarassment will be over.
I’ve accidentally tucked a dress into the back of my hose when in the restroom before. I did not get far before a woman I did not know yelled “stop!” before I got in front of anyone (in the little hallway thing before the bathroom door). It saved me so much embarrassment and humiliation. I appreciated it so, so much. If she hadn’t done that I’d have been mortified, especially when I had to come back here and post about the people that did nothing to help me when my ass was hanging out. 
This happened to me once. I was waiting tables and a male co-worker noticed that a woman customer had tucked her skirt into her pantyhose. This was at a buffet, so there was maximum chance for the woman to embarrass herself going back and forth. He asked my advice, being unsure about approaching her himself which might somehow come across as inappropriate. I suggested he discreetly approach the woman’s male companion. This seemed to work.
It’s an embarrassing situation either way. Tell her, so the embarrassment can be cut short. As for wording, try to not sound like some gross leering come-on and it should be ok.
I’ve seen this happen before, though it involved a construction worker and the way his fellow workers handled it was to surround him and sing “Moon over Miami” until he pulled his pants up.
That probably wouldn’t go over too well at a family restaurant, however.
With tongue.
Two words:
“Nice ass.”
What’s wrong with showing your ass in public? Britany Spears has made a second career out of it.
“It is not morning, but I can see the crack of dawn.”
OR
“The moon should not yet be out, and yet it is.”
They wouldn’t do that.
They’d use two warm spoons.
To add another lay of difficulty to the question, I’m a male high school teacher. Some of my female students are rather more…revealing…in their clothing choices than is appropriate. And sometimes a skirt rides up or a thong is showing or whatever.
My choices are pretty much:
- Find a female teacher quickly and ask her to tell the girl.
- Don’t say anything.
Do I think that if I discretely say, “Beth, you might want to fix your skirt” I’d be branded a pervert or lose my job or get sued for sexual harassment? Probably not. But I’m not eager to take that chance.
I guess it depends whether her name is actually Beth.
Since this was a restaurant, perhaps asking one of the wait staff to do the deed would be more appropriate?
You don’t even have to mention her ass.
“Ma’am, your dress” will do the trick. I would absolutely want someone to tell me and I’d absolutely tell someone if I saw it happening to them.
Yell “I see London, I see Asscrackistan!”
I don’t know why this is so much harder for guys - I tell people about embarrassing personal things all the time, but I’m a woman. One day I walked around for god knows how fucking long with my skirt tucked into my underwear before somebody finally told me. At LEAST thirty minutes, because that’s how long since I’d been to the bathroom. Thanks, coworkers. You’re awesome. (To be fair, it was a long skirt and you couldn’t see, er, Asscrackistan.
I generally just tell people and I’ve never gotten a negative response. As a guy it is sort of uncomfortable. I worry that women will think I’m a prev (Why were you looking at my ass!?) and that men will think I’m gay (Why were you looking at my crotch!?). So far, no bad reactions. Once someone figures out what you are talking about they got other concerns.
I once caught I guy in a motorized wheelchair coming out of the bathroom with toiletpaper stuck to his rear wheel, going round and round. He was grateful when I told him, we had a good laugh and I made a new friend.
Get a child, preferably a small one, to shout out at maximum volume, “Mommy, that lady is showing her woo-woo”.