Los Angeles
whoa that girl is thin!
look at her arms, id be afraid to snap them in half.
too thin for me 
There is another site (which I won't name, pretty sure that its against rules to talk about other review type sites) that provides the option for specifying ethnicity to a more detailed level than "Asian" (e.g., Chinese, Japanese, Korean, Thai, etc) The problem with having this sort of option is that it seems there are more people that cannot distinguish correctly between Asian ethnicity types than there are that can distinguish correctly.
Also there are exceptions to the "look" test. There are people who may look a certain ethnicity but not be that ethnicity. While some might argue that they can always tell just by looking, I would disagree with this. I might say that someone can be correct more often than not on ethnicity but being 100% correct all the time doesn't seem realistic. But that is definitely a different thread.
Overall, poor ability to judge more often than not and exception cases would lead to having a more detail ethnicity option as a value-less attribute in the searchable review criteria.
But yes.....keep on with the J-girl info
I am the OP and just want to note that this thread was not initially created to discuss differences, only to find out if people know of Japanese providers. 
My SO is Japanese and I spend time in Japan from time to time so I certainly can tell. Of course I won't insist my Jap-adar is not 100%, as no one's really is.
In my personal opinion, people can tell overall what nationality a girl is if they have experience with the cultures.
* Accents, pronunciation of words with R and L for example
* Height, body type, teeth, all can play a factor but are not 100%
* Shape of face for sure is visible, but not 100%.
* Shape of eyes can sometimes be used.
* The way people carry themselves
* If they speak English some of the the things they say can be used to determine where they are from, like linguistic phrases that are obviously translated directly.
* Their general responses to questions and how they handle certain situations. This is cultural and is not the same across the cultures.
* Fashion! this is a big one... You will rarely see Japanese or Taiwanese girl not wearing most of the latest fashions. Mainland Chinese girls tend to wear sweatshirts and very unappealing clothing. I don't know about Koreans, since my experiences with them are limited. Again these are not 100%, but just noticeable differences "most of the time."
* Makeup. Big one as well. Japanese girls have a very different way of doing makeup than Koreans for sure. Koreans tend to really cake it on IMO, making there faces very white sometimes. The way the eye makeup is done is also very noticeable.
All these are simply factors. And no combination of these will get you a 100% correct determination of the girls nationality. I can tell about 80-90% of the time just by passing a girl on the street. My wife is odd, people usually can't tell. Koreans think she is Korean, Chinese think she is Chinese, and Japanese think she is Japanese.
In the end, this is not what this thread is about. Thank you to those who have kept this topic on thread and linked to excellent Japanese providers. I am still in search of more unicorns...
TOFFT on Haru on CV. Pics are NOT her, contrary to what I was told. She's half K and J. Speaks J. Mannerisms are J. OK session but nothing to write home about. I've had better.
I've also seen Sakura under a different name. She's also a mix. Speaks J though. Really hot looking girl, service is passable. Sorry, my expectations are SO HIGH with k-girls now, I've been really spoiled with some stellar girls I've experienced over the last few months. . . . .