So here's my take:
I believe that every adult has a responsibility to be observant and to look out for the children they come into contact with, because if we don't, who will? Obviously you have to evaluate the situation. Sometimes you have to accept that, even though you don't agree with something, it probably won't really harm the child. Other times you have to be careful because, as some have pointed out, confronting the parent could make things worse for the child.
It saddens me that there are still so many people who will see signs on abuse and look the other way because "its none of my business", or because people would rather err on the side of not angering the parents. It also saddens me that there are so many people who still don't realise the bruises and fractures heel much quicker that the wounds words can inflict. Verbal abuse is at least as damaging, if not more than physical abuse.
I tend to confront people - even strangers who scream at their children in public. I have not come across a situation where I suspect someone I know of abuse, but if that ever happens, I will involve the authorities. I could never watch a child suffer.