Make friends with both Chinese and Japanese people... you'll soon be able to tell the differences very easily. And if you can't, they'll be happy to remind you. ;)

There are loads of ways you can tell. I can usually tell the difference in less than a second. The most obvious way is to listen to how they speak, but if they are not speaking, then you have to judge them by their appearances. In my experience the Japanese are generally much better dressed than the Chinese, and the Japanese will usually appear more refined (for example, better groomed, more youthful looking). If they are in a group, the Japanese will tend to bop up and down a lot and bow their head often, whereas the Chinese do not. The Japanese are also much quieter than the Chinese who can be extremely loud! But if you have to tell from just their clothes and the appearance, then the best dressed one and more refined-looking is probably the Japanese. The Japanese tend to wear more colours and more interesting styles, and the Japanese tend to be a bit slighter in frame. It's a lot more complicated than this, of course, but you would probably need to spend a bit of time in China and/or Japan in order to get a good feel for this.

From my experience, Japanese speak faster than Chinese which means when a Janpanese speak, his or her lip would move much more frequently. In addition, when Japanese talk to people, their hands will not in pocket and their eyes will not drift away, while some Chinese may. Hope this helps

Mainly from the language they speak and their accents. Just like you distinguish between a German and an Englishman. Then there is the body language, the shape of the nose, the eyes, the overall charisma, blah blah blah. The list goes on.

Chinese and Japanese language is completely different and completely unrelated. If you know one you can spot the difference in a second. For people, you simply can't.