I have to admit that when I see a child tha looks to be a different ethnicity than the parent, I assume it’s adopted and you know what happens when you assume! Having a niece and nephew that are half caucasian half Japanese, I should know better. But I would NEVER ask!! But do agree, it’s all in the delivery as to whether it’s offensive or not!
I personally get asked this question a lot because both of my children are half asian. I do not take offense to the question because there is a good reason why people ask me. I think is better to just not ask because you never know if someone might take offense. I think it all depends on the parent of the child in question. To me it is no big deal but to someone else it could be very offensive.