I've consolidated responses to your two responses.

Belldandy has a "western" figure?  Could've fooled me.  I 

thought this
was

Urd until they got to the temple.

Western?  Yeah, of course.  She's a germanic goddess, you 
know, and she
certainly isn't supposed to look Japanese (I believe I recall that
Keiichi's mother calls her a gaijin, but that might have been 
a fanfic)...

Belldandy certainly has a western _face_.  Everybody knows she's
a gaijin at a glance.  By western _figure_, I assumed you meant
hips and bosom.  Urd is drawn this way, but Belldandy is always
drawn with quite modest proportions, about what the Japanese
girls are drawn with.

Also, <begin soapbox>, Japanese and Chinese people can't 
reliably tell each other apart
until they open their mouths.  There is no difference in 
"slopes and lines around the eyes and nose" consistent 
enough to use as a distinguishing feature. <end soapbox>

Not really true- I'm not asian, and I can tell them apart
rather easily (though I have trouble sorting Koreans,
Vietnamese, and Chinese).  The thing I notice most is the
eyes and nose, though there IS something about
the facial hair that I haven't caught yet.  My mother,
however, is able to catch all these things easily...).
Perhaps there's no 'official' biological indicators, but in
practical experience I have to say I can notice it.  I do
wonder, though, if Mr. Jonathan Desaix (who knows almost
nothing about Japan or the Japanese) WOULD be able to tell
them apart.  Well, I say he can (or at least that he thinks
he can), so I guess he does... *grin*

I received a number of responses with persuasive anecdotal
evidence that I was wrong on this, at least when Japanese
or Chinese people, or experienced westerners, are _thinking_
about the question w.r.t. a specific individual.  None of the
respondents seems to be able to say precisely what the
difference is, and the difference also seems to be statistical
rather than deterministic, i.e., there is an error rate.

For purposes of your story, I'd say that Jonathan can think
whatever you want; after all, he has a 50% chance of being
right.  (You can plausibly have him able to rule out Korea
by shop signs, by having him live close to a Korean church
or restaurant in the U.S.  The Korean character set looks
very different.)

For other fanfics, the evidence indicates that Qinghai
shopkeepers are far more likely to overlook their silent
Japanese customers entirely, or at most to comment that
they "almost look Japanese" with a chuckle, rather than
shout out, pointing, "Ahhh!  Japanese!"

Akane Tendou-san has placed an ad

In English, this is strange.  Belldandy hasn't explained 
"san" yet, and she's using our name order, so she should
probably be using a prefactory
"miss".  I hope she does explain Japanese name order before 
Akane gets there.

No, Belldandy's not going to explain honorifics... I think 
that's going to
be one of the more advanced lessons, actually- it can get 
rather confusing.

Well, it's one thing to explain proper usage of all of them;
it's another to simply tell him to append a "san" syllable
onto names so he doesn't offend people, and he knows what the
"san" they use for him means.  When I studied Japanese, "san"
was covered fairly early, with the comment that, as a gaijin,
I would seldom if ever have to use or know any other forms.

Also, my other original point was this:  since Akane's going to
say "Atashi, Tendo Akane desu" when she gets there, your
character is going to be confused if Belldandy doesn't explain
that the Japanese reverse the name order.