I sometimes find it difficult to tell between Japanese and Chinese. However, sometimes I can distinguish the differences with the way they act. I find that Japanese people tend to use more body language and are more polite when talking whereas in my opinion, the chinese are more loud. Japanese girls seem to act more cute which I think is part of their culture as well with the whole Kawaii theme going on in Japan.
I asked other people whether they can tell the difference between chinese and japanese.
The majority that said yes was asian and commented that you can see a difference in their eyes and skin tone where the Japanese have paler skin. They also mentioned that they can sometimes just sense whether they'r japanese not. The tone of voice is a lot different to the chinese and also the accent when speaking in English, similar to what I said about the way they speak.
Non asian people finds it a lot harder to distinguish the differences between the Japanese and the Chinese.
I'm chinese and I still find it difficult to tell the difference at times. But that might be because I have not been around Japanese people much at all.