I am Khmer but by dad is unsure of his ethnicity (he moved to the city all by his lonesome self when he was young)--so I am definitely half Khmer--I think. We went on a trip to Cambodia last year and in his village the people there speak Lao. My aunt told me my gradparents are Laotian but I told her our last name sounds Khmer to me.
Any who, I do get once in a while "Are you Lao" from fellow Cambodians. I live in a majority Cambodian city--if you are asian of course. I've gotten quite a range from both asians and non-asians. I've been assumed to be hispanic, Indian-mixed, Thai, Chinese, Lao, Korean (only once though--I was in Cambodia), pinay, or khmer-mixed.
I'd be proud to represent all--it really doesn't matter to me. I don't think it is that easy to identify ethnicity.