How can Chinese people tell each other apart?

No. But if you observe in more details, you may find that their behaviour, their manners, and of course , their languages, are different.

^ such an ignorant answer. You can very much so tell the difference between a Japanese girl and a Chinese girl...you just have to know what to look for. Without getting into too many specifics, Chinese women will have striking features that segregate them from other Asian women...eyes have more drastic angles, cheek bones are slightly higher, etc. Japanese women tend to have slightly more almost shaped eyes and softer features. I'm an Asian and can tell almost every Asian apart from another with very few exceptions...just because we're Asian doesn't mean we all look alike.

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Let's be fair to the original answerer here... While there are certainly various typical differences between Chinese and Japanese women, these differences are not present in 100% of cases, nor does their presence indicate a guarantee of one ancestry over another (meaning that just because a woman has almond shaped eyes and lower cheekbones for example, does not guarantee that she's Japanese).

People jump on the racism train pretty quick these days, but just because someone can't tell if a person is Chinese, Japanese, Korean, or whatever, doesn't mean they "think all asians look alike". Especially when you yourself (second answerer) admitted that there are times when even you can't tell. Heck, I was in high school in Japan for a year and had a Taiwanese girlfriend who was mistaken for a Japanese girl constantly, by Japanese people.