Well to find out if the girl may be Japanese, some of the bros have suggested.
a.) Speak to them in Japanese.(This is the easiest way to tell if you yourself have Japanese language skills.)
B.) See if they use any Korean or non-Japanese electronics. (ie. Samsung cell phone) According to some people
a real Japanese girl would never use Korean products. (Not sure how true this is.)
Having said all this, as long as they are hot, who cares
If you can't tell, then your fantasy of f$cking a Japanese girl is still intact!