Well, Let Me Guess...

I work part-time at a Japanese bookstore. The bookstore mainly carries Japanese books but they have some English books and Chinese books too. So probably the biggest ratio of the customers would be Japanese people, but quite a bit of non-Japanese come to the bookstore, especially Chinese people. Actually the bookstore has a stationery section, and apparently Japanese stationery is pretty popular among people here so not only Japanese, Chinese and Americans (I'm not totally sure what I mean by "Americans" tho... like white people?) but also people of other ethnic groups, like Korean, Indian, Hispanic, Vietnamese, African, Russian, French, Persian, etc. (these are actually the customers I've taken care of,) come to the bookstore, and of course, they speak English to communicate with me.
At a cashier counter, when I ring up a customer, I always think that it would be easier if I always talked to the customer in English. But, in reality, I can't. To be precise, I don't want to. Because If the customer is Japanese, I don't want to talk to him/her in English, like I talk to a Japanese customer in English and he/she answers in Japanese, then I start talking in Japanese... well, I mean it's totally fine, it's not a mistake, it's not rude at all, but it just feels kind of stupid, especially when a Japanese customer answers in English and force him/herself to keep talking in English. It's probably because my English is not as good as a native English speaker's and the customer's English isn't either. So it's like two Japanese people trying to make some non-native conversation. It just feels funny. It might be fun to do something like that with your friends (actually some of my Japanese co-workers talk to each other in English for fun,) but not with a customer.
Then, what can I do with it? I basically just need to deal with it because it is very hard to guess perfectly whether an Asian customer speaks Japanese every time you see an Asian customer. But I still can guess! There are some aspects to help my guess.
The stuff a customer brings to the cashier counter is a very simple and easy indicator. If an Asian customer brings a Chinese magazine, I would just talk to him/her in English. If an Asian customer brings a Japanese novel, I would just talk to him/her in Japanese.
Appearances are very important, of course. Hairstyle: there are some particular hairstyles some particular ethnic people seem to like. My co-worker has told me that if you see an Asian girl with long hair partially dyed in gold, that girl is probably Korean. Facial Parts: a person with round nose, round eyes and double eyelids are good signs that increase the chance that he/she is Chinese. Outfit: it's kind of hard to use outfit to guess where an Asian is from, but I use it more for distinguishing American-born Asians from Asian-looking people. For example, the outfit of Japanese international students and Hong Kong international students look alike, but the outfit of Japanese students who grew up in the US tends to look pretty different. Makeup: I don't know how different they are but I feel like makeups are pretty different among different ethnic groups. It's hard to explain, but it gives me some hints.
Behaviors make them look different too. Facial/Verbal Expression: people who grew up in the US seem to smile at a cashier more and to say something to a cashier more than the people who grew up in Japan. Posture: I'm not so sure about this but the way they stand and walk gives me a slight hunch... I think.
So these are just indicators and factors I use to guess whether a customer speaks Japanese. I do not mean that these are the traits I use to determine an individual's ethnicity, but to guess. Cultural influences are probably not the only factors which shape an individual's characteristics (especially for appearances, of course.) And the influence itself wouldn't be so simple anyway. But I believe that it works not bad.