A few months ago, I was finishing up my workout with a plank when a woman in purple leggings approached me. “You’re going to hurt your back,” she said. “Do you mind?” Before I could respond, she put her hand on my lower back, adjusted my form, said “there you go!” and walked off. I sat up a little bewildered. Who was she? Did she work here? Did she even know what she was talking about? All unclear.

Sure, a helping hand can be appreciated, especially when someone is new or seems confused. But where is the line between offering advice and flat-out intruding on a stranger’s space?

Personally, I can’t imagine ever approaching a stranger at the gym to offer advice, but hey—I’m not a professional or an expert. So to find out if Purple Leggings’s behavior was as bizarre as I thought it was, I spoke with a handful of personal trainers. Unsurprisingly, some took an “only when I’m on the clock” approach, whereas others insisted that offering unsolicited advice is just never a good idea, no matter the circumstances.

When in doubt, remember this wise old proverb: “To assume is to make an ass out of u and me.”

Don’t assume anything about a stranger’s fitness levels. Nine times out of ten, when you think you know what’s best for someone without having the full story (um, this is true in any situation), you’re running the risk of offending them—or worse, hurting them.

“I tend to err on the side of not making assumptions and try to consider reasons why someone may not be performing an exercise the way I’ve been instructed to,” says Summer Pierson, a discus thrower and Highland Games champion.

Andrew Freeman, an L.A.-based trainer, agrees. “Someone may be modifying an exercise for their own reasons or because they were instructed to, and assuming you know their goals or their body is never smart if they’re not your client.”

But what if someone is in obvious danger of getting hurt or hurting someone else?

The truth of the matter is that this is why liability forms exist. Gym members sign a waiver claiming responsibility for their behavior in the facility—so if you’re not an employee of the gym, it’s not necessarily your place to play the hero, despite your good intentions.

Jaime Rodgers, a personal trainer in Cape Cod, MA, says that if it’s an unsafe situation, it’s best to ask someone with authority at that gym to step in. “I’ll go say something to the trainer I know they usually work with—that way they can be more personal about it.”

But Freeman adds that seeing someone intentionally using equipment wrong is sure to elicit a quick response. “If I see someone goofing around on the machines or doing something dumb, I’ll definitely tell them to knock it off, whether I’m working or not. No question.”

It’s all about the approach.

Some trainers feel there’s a polite middle ground where they can be helpful and instructive without being intrusive (or making someone defensive). The pros agree that if you’re going to approach someone, it’s important to introduce yourself as a trainer and always ask before stepping in. “I open with a question like, ‘Do you have any questions about using this equipment?'” says Kate Browne, a fitness instructor in Bloomington, IL. “That gives people the option to accept your advice without feeling defensive about a critique.”

Use your best judgment and take a second to observe the person before jumping in to save the day. If they look like they spend seven days a week at the gym, odds are they think they know what they’re doing and don’t want your two cents. On the other hand, if someone’s wandering around aimlessly and wielding a kettlebell like a battle ax, it might be safe to strike up a conversation to see how they’re feeling about the equipment.

“Usually it’s appreciated,” says Chelsey Hughes, a trainer in New York. “Someone has to be pretty prideful to refuse guidance from a veteran or expert.”

Bottom line: If you’re not a professional or an expert, you should generally leave others alone at the gym.

If you have genuine concerns about their safety, discreetly ask a staff member to intervene—even if you’re a fitness professional, deferring to someone with authority at that particular gym is still the best way to go.

And of course, the one piece of advice every trainer I spoke to agreed on: Never put your hands on someone else unless you have asked and received their permission (but that’s just good life advice, to be honest).

I never saw the woman who corrected my plank form again, leading me to believe she was just a well-intentioned passerby worried about my lower back. But for those of us non-pros out there, even if your intentions are pure, the best practice is this: Unless someone is in immediate danger, when it comes to a stranger’s form at the gym, just live and let live.

Laura Munoz is a freelance writer and distance runner living and working in Los Angeles. You can find more of her work at lauramcreative.com.