Fron my own experience, the only closest, reliable way to tell is to look at their grooming n dressing.
An alternative is to listen to how they speak english.
Visual-
Chinese: dressing less trendy, no dyed hair. For mainland chinese men-usually short, square-top hairstyle.
Korean/Japanese: fashionable, usually bright colourful clothes. Dyed hair
From speech(assuming speaking in english). This is easy n i accurately tell a person's nationality just by hearing his/her accent in english.
Singaporean Chinese: very deep-toned. full of affixes of 'la', 'lo', 'lei', 'ah~' at the end of sentences. This is due to the heavy Hokkien dialect influence, which is the lingua franca Chinese dialect in Singapore n is naturally present even in those that do not speak it. Malaysian chinese, even though their lingua franca is cantonese- tend to speak the same way too.
Hongkong Chinese: very high-pitched n also full of some of the affixes found in hokkien. its because Cantonese is the lingua franca chinese dialect. Watch youtube n listen to how hongkong celebrities speak in english n u will know what i mean.
Taiwanese chinese: exaggerated, highed-pitched, unatural american-slang-ridden english. Maybe due to american media influence. But when they speak in Mandarin(which is full of 'oh' affixes at the back, i can str8 away tell they r from Taiwan.
Mainland Chinese: awkward, high-pitched english, maybe due to anerican english thought in schools. they tend to break words into individual syallbles n pronounce words beginning with 'V' with the 'W' sound(maybe due to no 'V' sound existing in Mandarin). E.g Victoria will be said as 'Wee-toh-li-ya'. When they speak in Mandarin, its more 'standardised' with 'qiao she yin' n some 'e yin', unlike the dialect-influenced Mandarin here in Singapore.
Koreans/Japanese: high-pitched n almost same as mainland chinese. The Japanese tend to pronounce words beginning with 'R' with a short, slight rolling sound like the 'R' in spanish.