I usually don't say anything, unless the child is in danger (from either negligence or "over disciplining"), or harming my child.
It's funny you posted this--I just had this conversation with my mom.
I doubt I would ever say anything. I've wanted to many times-but never thought it was my place. The only time I will say anything to a child that isn't mine is if what they are doing is harmful to me or my children. My neighbor (ex-friend) was always correcting the other kids in the neighborhood & I could tell that the other mom's just didn't appreciate it (me neither).
Several times I've thought about calling the police when I've seen kids in cars without being restrained-but have never done it...
I am also a very non-confrontational person although I am speaking up more the older I get.
This question brings up a sore point for me. When I first quit my job to SAH I met a group of friends and we would hang out all the time. At that time the kids were 2.5 and 6 mos, or thereabouts, and the discipline styles didn't really matter.
As the kids got older (especially the older ones) it became hairier as the differences in parenting styles started to come out. I found myself having to explain my own parenting choices to Connor. He'd ask me "why can so-and-so climb on the furniture?" or "run screaming in the restaurant" and so on, and then the other kids started getting pretty nasty toward him and the parents never reprimanded them. In short, there was a falling out with a few friends and I stopped hanging out with them...but I never confronted them about the real reason (the lack of proper boundaries for their kids). I didn't see the point; they'd get defensive and nothing would come of it.
At this point, I prefer to have a few close friends whose parenting styles match my own. I do still have one friend who is pretty lax with her kids but when we get together I just leave her kids be and let her handle them, unless they're in danger. It's not my business to parent someone else's children, and I'm not interested.
I don't say anything. I would be appalled if someone in a grocery store said something to me so I wouldn't do it to anyone else.