The nice ones try to do it in a nice way...the backhanded Southern way:
"I bet you didn't see there's not a hechsher on that bag of spinach."
"Excuse me, but the cholov yisroal is located over there."
"I'm sorry to bother you, but your bangs are showing."
"You must be a newlywed! Let me show you how to cover your hair correctly."
"Bless her heart..." has been ignored because it's normally said behind the apikores' back.
Not all people are capable of being "kind" when questioning your halachic observance.
"Don't know you that Jews aren't allowed to own dogs?" (My personal addition to this list)
"Your hair is showing."
"Don't buy that; it's cholov stam."
"There's no hechsher on that spinach. How are you going to check it?" (Not just will you check it, but the person wanted to hear the actual procedure outlined!)
"Where are your tzitzit?" (Just so we're clear, you can wear them inside your pants, and many people choose to, especially for employment reasons.)
"How many sets of dishes do you own?" (Apparently the answer was supposed to be 3.)
Only heaven can help you if you buy fresh, un-hechshered broccoli or cauliflower. For three years, I didn't eat it because I had been told you can't check it and thus, it is treif. I still only eat it if other people fix it. I think I have PTSD for cruciferous vegetables...guess it was growing up around all those crosses in the South.
And the classic:
"That's an aveirah." (Catch-all with deadpan delivery)
Speaking of the war declared on fresh produce, I want to clear something up here: you need to check ALL bagged or fresh produce, hechsher or not. The only complaints I have ever heard about bugs in bagged salad have come from hechshered bags. Personally, I will check hechshered bags far more thoroughly than a national brand bagged salad. FDA standards and consumer demands seem to trump the heimish companies in this respect. (And that's another reason I'm sticking to FDA cholov stam. Other than the fact that it is absolutely halachically acceptable and half the price for an unemployed lady.)
My beef with these comments isn't the content of those comments. There is often a halachic reason for the rude person's belief (tenuous or unjustified though it may be) ...But is there any justification for approaching a stranger and rebuking him for someone that is clearly accepted in the orthodox community? Even if it does appear out of character for the community, do you really know all the reasons or actions the person is taking? And do you even know the person is really Jewish, much less orthodox? A woman wearing a skirt in a grocery store in a Jewish neighborhood does not a Yid make.
I'll give you an example: sometimes I buy treif hotdogs. Yes, mamish treif, all porked up. Why? Because I have 2 dogs, and hotdogs are their Holy Grail. I'm not going to waste expensive kosher hot dogs on them when I can buy treif hotdogs for 89 cents for a pack of 10. Why can I buy them? As I was taught, I can buy treif food items for my pets (hello, cat food and dog food are all treif!), but you can't benefit from mixtures of meat and milk. That's harder, but not difficult. Most pet foods that have both meat and dairy will list it prominently. For example, Fancy Feast Cheddar Grilled Turkey is out for the kitty cat. ...But Cheddar Grilled Whitefish is fine. However, the average non-pet owner on the block doesn't know these things, and I've been approached about them. Of course, no matter how well you "explain yourself" (to someone who has no right to demand an explanation for your behavior), the best you can get is "Well...ok then...If you say so." Sometimes they tag on "But maybe you should double check that with your rabbi." You cannot convince a hater unless you're a rabbi. At least when you're a man, there's a chance you're a rabbi, so haters will often step more lightly. But women? All bets are off, especially as many men are becoming more vocal about the alleged inadequacy of women's halachic observance. Personally, I've noticed that women are overwhelmingly the victim of such halachic questioning by strangers in public places.
Does halacha extend the mitzvah of "rebuking your friend" to a person you've never met? I'm not a scholar, but my common sense says no, and I believe that it would actually prohibit such activity. When you approach a stranger, you see a very small sliver of his or her life. You don't know anything other than the physical trappings on the outside. You have a mitzvah to judge favorably, and you don't know the other factors of the situation. And no stranger, NONE, has the right to question a person about how she or he performs an alleged mitzvah unless it actually affects the stranger. (I say "alleged mitzvah" because customs-confused-as-halacha are also targeted for "enforcement".)
Only certain people have the right to know your understanding of halacha, and that includes, among others, Hashem, your spouse, your children, and close friends, your rabbi, maybe your local rabbi (depending on your rabbinic relationships). The stranger on the street (or even in the shul) has no right to know why you cover your hair with a tichel instead of a sheitel or why you show a tefach of hair or all your hair below your hat. Those people can certainly wonder, and they can even stereotype you and ostracize you for it, but if you are questioned, you owe them no explanation. Any explanation you give is voluntary. I want you to remember that because many of us do answer these inappropriate questions. You have the right to decline the conversation, and you have the right to spend 30 minutes answering their questions. (I fall into the "let's have a conversation!" camp), but recognize that it is your choice, not something you've been forced into. And never take the bait from someone who will never be reasonable or respectful of your Judaism.
I think that a last example will help show my point. A friend was riding the Egged bus while studying in a seminary in Israel. She was wearing a scarf or hat of some kind, with her hair hanging out the back. Let's set aside the fact that this style of haircovering is considered halalchically-approved by many rabbis and is the community custom for most Americans. While riding the bus, my young friend was verbally attacked by an older woman who accused her of covering her hair improperly and how this aveirah was such a harm to the Jewish people. This tirade continued for several minutes before my friend quietly was able to interject, "I'm sorry, but I'm not married. I just like hats."
Did that older woman feel like a jerk? I doubt it. Because she was trying to do the right thing.
But that's my point: that's not the right thing. This kind of behavior brings unnecessary pain to Am Yisrael and that pain drives people away from observance. And only heaven can help you if you've accidentally rebuked a non-Jew who looked Jew-y.