If you're a gambling man and consider the populations of each country, then your always getting the odds to go with Chinese
I'd just ask. It's not hard, and it removes any chance of offending the person or making yourself look stupid: you can't lose.
You have to be understanding to an extent, because we're given little news, TV shows, or general exposure to Southeast Asia, and in a lot of areas, there's slim to no representation in the population. This means most Westerners don't really have a profile for individual countries, and everything is defaulted to Chinese
Some people
are a bit "culturally unenlightened" though; there have been a couple who assumed I could understand Chinese and like Thai curry because I'm learning Japanese
Thai curry is nice though
Regardless, it's rarely easy to say for sure, I find. You do get "typical" Japanese/Chinese/Korean/etc faces, just like you get typical Polish/Czech/Croatian/Russian/etc faces as well, but outside of those, I'm sure even people from a given region would have trouble distinguishing their neighbours on looks alone.
That's my little bit of sociological insight anyway