“The Rules” is a Moneyish series where we define the rules around sticky money or workplace topics like giving an allowance, who pays on a date, combining finances with your partner, and more.

Psst: Your fly is open.

Made you look — but chances are you’ve been caught before with kale in your teeth, a lipstick-stained tooth or schmutz in your hair while at work or out with friends. Even worse, you’ve probably had to notify some poor soul of their own awkward makeup or wardrobe malfunction.

Take Emma Watson, who took it upon herself last year to wipe a pen mark off a reporter’s face: “You would do this for me, and I would do this for you. You’ve got pen on your chin, and you’re going to be so sad when you see this, and I just have to do this for you,” the actress said, leaning across to the on-camera interviewer. “I’m so sorry; I know what it’s like to have something on your face.”

So when is it appropriate to extend a helping hand? Largely in scenarios “where it’s something obvious but obviously correctable — and, unfortunately, it’s obvious to everybody but the person who’s had this slipup,” etiquette expert Thomas P. Farley told Moneyish. “I think those are the ideal conditions for making a correction.”

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Key factors to consider, said author and etiquette expert Lizzie Post, include “time, place and degree of embarrassment.” Of course, there’s no blanket advice for each and every scenario; for acquaintances, coworkers and friends. “We all make the choice in the moment based on all the minute circumstances involved in it,” Post said. “The personalities of the people involved are so at play here.” Still, here’s expert counsel on how to navigate many of these situations:

Get over your own hesitation. Sure, you’re dreading pointing out someone’s unsightly imperfection — but put yourself in their shoes. “If you feel like you would want someone to tell you, then do it for someone else too,” Farley said. “Nobody is going to come back with anger … because you’re pointing something out that could potentially embarrass them. They’re going to be grateful.”

Etiquette expert Elaine Swann likens the act to “working a muscle”: “The more you work that muscle, the stronger you build it,” she told Moneyish. “I challenge individuals to look out for other people, and speak up when you see something is wrong, and do what you can to try to be helpful.”

Do it discreetly. “You don’t say it in front of a full table of people,” Farley said. Get within their earshot, he suggested, or catch their eye and gesture “that universal symbol of ‘You’ve got something in between your teeth’ that most of us recognize.” “We’re always looking for some kind of comic relief at work, but don’t make it a thing,” advised Swann. “Don’t go and make a workplace pool on who’s going to go tell so-and-so about what’s wrong. Just make a decision as to whether you’re going to do it or someone else.”

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Be kind. Farley says reality star Bethenny Frankel did him a solid while he spoke at a September 2015 “petiquette” event. “She just stood up and said, ‘I’m sorry, I don’t mean to interrupt, but we’ve got all these cameras here … I can’t let you continue with your bowtie being crooked.’” Frankel’s intervention was “done in such a loving, caring way,” Farley said, that he didn’t feel bad about it. “She probably put voice to what at least a good chunk of the people in the room had been thinking,” he said. “I was thrilled she did it.”

“You just want to have a gentle tone … not something disparaging, or not something that makes the other person feel like slob,” Post added. “This stuff happens to all of us, you know?”

Keep it simple. Post recommends starting off with “I’d want someone to tell me…” Farley advises, “‘You’re probably not aware, but…’ and then fill in the blank.” “There is no way to take away a person feeling embarrassed by a mishap,” Swann added, “but you can take away the sting of it by not spending a lot of time pointing it out.”

If you’re dealing with a massive power differential or someone with a big ego, Farley said, it may be best to stand down. “Say it’s your boss’s boss’s boss and you’re the low person on the totem pole in the organization — I might not want to be the one to approach the person. I might leave it to someone a little more senior.” If it’s that obvious, he added, someone else will notice and point it out.

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Ask before touching. Work within the other person’s boundaries of personal space, Post said: “If it’s the food on the face or something on the hair, you don’t reach out and help them without first making the offer to do that or without them first asking,” she said. “It might feel really natural to just reach up and pluck that piece of paper out of their hair, but … just be careful and don’t immediately try to fix something on a stranger or on a friend.”

Consider the gender of the person with particularly sensitive fixes like an open zipper or a too-unbuttoned blouse. “Before jumping into pointing it out, I would consider whether it would be more appropriate and better taken coming from the same sex of the person having the malfunction — just so there’s no danger that your comment is going to be misunderstood or taken in a spirit other than what you would intend,” Farley said. “Give some thought about how the recipient who has their zipper down is going to receive the bearer of the information, and make your decision based on that,” Swann added.

Use your judgment, especially with people you don’t know. With a stranger on the subway, for example, “it’s really up to you and your comfort level,” Post said. Plus, she added, it’s good to remind yourself that “you have no idea who this person is.”