Are there any scientific ways to tell the difference between a
Japanese vs. Chinese person?

Ask them. If that isn't scientific enough, ask them during a polygraph test.

Here's an article that may be of interest:

http://www.ncpa.org/pd/social/pd082300d.html

For a more humorous look at the differences, or similarities, I direct you to:

http://www.alllooksame.com/

which has an online quiz asking you to try to classify by race
different pictures. I only received a 8/18 which is almost
statistically the same as random guessing, and I'm Chinese! But give
it a shot and see if you can best the 7/18 average.

skermit-ga

I scored 2/18 on that quiz--significantly worse than random guessing. 
And my in-laws are Chinese!  I eagerly await someone answering this
question.

Additionaly, 

To get a statistically significant scientific answer ask them at least 30 times.

well, here is the interesting thing. I go to a Japanese / English club
and there was were 2 japanese guys and the one asked the other "What
generation Japanese are you?". It turned out the second guy was
actually chinese, so I'm not so sure that they can even tell.

However, Isn't it interesting that they can get offended if you 'guess wrong'.

I've never gotten upset with a friend or even stranger who's asked me,
hey, are you (insert any Asian nationality besides Chinese... I've
gotten practically all of them). After all, it's just a guess, and I'd
be hard pressed to determine an western european's descent or which
country a particular south american came from. I do get upset with
people who take that to mean that similar looking peoples share
similar culture, food, customs, etc. by assumption and ignorance.

skermit-ga

The Japanese have slightly sharper noses than the Chinese according to
my observation.The japanese are towards the wheatish side when it
comes to complexion.We can just guess based on these criterias.To be
100% sure it is always better to ask them!

From my observations (not a 100% thing)

Chinese are typically lighter in complexion
Japanese have bigger noses (and god they are sexy ;) )
Chinese have wider cheak bones

I usually wait for them to say something. Since I'm learning Japanese,
it's rather easy to determine if the spoken language is Japanese or
different Asian language.

Josh

I'm the president of the international club at my college and I can
only really tell if someone is japanese or non-japanese. The problem
lies in history. The ancient culture that the current Chinese
government credits as its ancestors is that of north-eastern China.
Koreans also cite these people as their ancestors. In fact, about six
months ago Korean leaders were enraged that a Chinese city held a
festival honoring these early people. For many years Korea was
dominated by Japan, so now there are people in Korea with Japanese
heritage and a lot more people in Japan with Korean heritage. My
friend Toshiki has Korean heritage on both sides of his family, but is
very much japanese. He looks Korean in complexion and in the face, but
dresses and speaks English like he's japanese. With Korean spread
everywhere, you can't be sure anymore. Listening to them speak,
Japanese often pronounce "railroad" as "lail load" or "rail load"
while Chinese sometimes pronounce it as "rair road". This is due to
the differences in languages. If you can here them speaking their
native languages, Japanese is something that an American can usually
pronounce, while candonese is much more like a goose talking. Mandarin
is not as bad, but i don't urge anyone to try to learn it. I really
know very little about Koreans. I've come to the conclusion that a DNA
test isn't even a scientifically valid way of determining someones
citizenship in Asia. Your best bet is to ask for their passport. If it
says China it may still be incorrect, as a friend of mine obtained a
legal Chinese passport illegally with a small bribe.
Matt

for scientific ways, the data used for telling the difference should
not his/her picture, check the accent, passport, parent.....

after all, chinese believes the ancesters of japanese are those lived
in ancient china in Qin Dynasty.

For scientific ways, the data used for telling the difference should
not his/her picture, check the accent, passport, parent, etc.

BUT the ancestors of Japanese are those lived in ancient china and
they were fishermen travel crossed the sea and stay there.  And then
around the Tang Dynasty, the government of the time have missioners
visit the islands and take as colony.

Of course, after years of develop, they invaded countries during the
Second World War.  They were raped women in different counties, killed
people, and done many evil things.  Of course they always denied what
they have done and never educated that to the new generation of
Japanese.

Anyway, back to my point, it is hard to tell if you are not either
Chinese or Japanese or Korean; but if you are, you can just tell by
their facial structure.  Just like, how Mexican can tell between
Mexican and others.

I can't really tell by looking, but if you listen you'll notice
differences of the word 'umm' in languages:

English-Ummmm....
Korean-Cheeee....
Japanese-Etoooo....
Chinese M/C- (not sure)