...or is there something else that hinders her power of discernment?
I'm a white American guy, and even I can tell Japanese people apart from other Asians. To put it bluntly, most Japanese persons look like the hybrid offspring of a pair consisting of an East Asian and a southern European (Greek or Jew?). That's not to say that the Japanese are closely related to any extant European population; in fact, I am almost certain that they are very distant from each other in terms of line of descent. The fact remains, however, that morphologically at least, Japanese people have a relatively high incidence of certain physical traits that are extremely rare among other East Asian populations. I think this is likely to be due to "shared retention" by the ancestors of the Europeans and the Japanese aborigines of a certain suite of physical traits that was typical of the original Eurasians, while the ancestors of continental East Asians underwent several severe changes to their skeletal (and especially facial) morphology during their most recent stage of evolution. This scenario seems to be supported by the fact that ancient skeletons of modern humans found anywhere in Eurasia, and even the oldest human skeletons found in the Americas, all appear to possess rather Caucasoid morphology, and skeletons that exhibit the prototypical features of the Mongoloid race appear only later in the archaeological record.
I rarely have any difficulty distinguishing Japanese from Chinese or Koreans, but the continentals can be a troublesome bunch.
The unusual features of the Japanese that set them apart clearly in most cases from any of the continental East Asians are:
1) Japanese tend to have a more pronounced facial topography (i.e., a rather "bumpy" or "projecting" look to the face, such as around the eyebrows, rather than the smooth and flat contours of Chinese or Koreans)
2) Japanese tend to have a more perceptually salient nose. This can be either more salient in simply the degree of projection from the surface of the face, or more salient in terms of the total volume of the nose (i.e., including the width). In general, Japanese seem to exhibit a much greater variety of nose sizes and shapes than do continental East Asians, and I have met many Japanese who even have "bumpy" noses with several bulges and constrictions in the contour of the nose, as I have otherwise only observed in Europeans. Chinese and Koreans appear to have only smooth-contoured noses, regardless of whether they are flat and broad (as is common in southern Chinese, Vietnamese, etc.) or narrow and slightly projecting. This feature is particularly relevant for distinguishing Japanese men from Chinese or Korean men, because women of every nationality tend to be rather paedomorphic when it comes to their noses.
3) Japanese people tend to have a diminutive lower facial region. They often have small jaws, which may be the direct cause of their propensity for having poor alignment of the teeth. Continental East Asians, on the other hand, seem to have huge jaws, flaring malars (cheekbones), and a generally large and imposing lower face when viewed from a Caucasian perspective.
4) Japanese people often have rather translucent skin, similar to that of Europeans, when they are not tanned. Therefore, Japanese people often have rosy cheeks and a generally healthy-looking complexion. When they do tan, they tend to take on a reddish-brown color. The Chinese and Koreans, on the other hand, are almost all cream- or beige-colored ("pasty") from the start, and they have completely opaque skin, so that it is impossible for them to have rosy cheeks and they always look sort of sickly unless they are tanned, in which case they take on a yellow-brown color.
As for distinguishing Chinese and Koreans by sight, I think it is quite difficult, but not impossible. They both have a propensity for big faces with a smoothly rounded outline, but Koreans tend to be more extreme in the width of their faces, so that they often have a nearly circular look, whereas Chinese tend to have more elliptical faces when viewed directly from the front. Chinese also more frequently have double eyelids and larger eyes that seem to bulge out of their (flat) sockets. Korean people tend to have very small eyes and no eyelid creases. Among East Asians, Chinese people also have a peculiar tendency towards prognathism, so that they often have bulging mouths that look somewhat reminiscent of black Africans. The big, bulgy eyes and mouth that appear so frequently among Chinese people seem to me to suggest some sort of affinity with populations of Southeast Asia. Also, I'm not totally sure about this, but I have a hunch that Koreans more frequently have a sort of oily shine to their skin, whereas Chinese people's skin tends to be more dull and dry-looking.