Well, people ask me my nationality and assume I'm more than one ethnicity by looking at me, which is perfectly fine with me. It's just modest curiosity. I don't see how this question should ever be interpreted as offensive, even if asked by a total stranger, because honestly, it's not personal. People shouldn't need to warm up to you before asking your race. It's just trivial. I have modest curiosity myself and ask the heritage of others at times - politely, mind you-and none of them have ever taken offense. And even in the case of assuming certain features belonging to certain races and vice versa, there should be some patience applied, because it's not an ignorant way of thinking really. It's pretty unavoidable. Just let them know that every race can have any conceivable trait and move on.