Guys have any thoughts on how I can tell if she's a gaijin groupie or not?
Ask her to introduce you to her parents.
This is a major complaint I get from the more honorable gaijin friends I have in Oita. They find lots of girls who want a gaijin boyfriend, but the minute you ask them to introduce you to their parents, you get the cold shoulder. Guys who grew up with the code that it is only honorable to date a girl with the approval of her parents have a hard time in Japan. We do not have a tradition of the date where a guy escorts the girl from her front door and back to her front door. You don't usually meet a girls parents until you go for the "goaisatsu" to anounce your proposal of marriage to her father. So you have to explain to her that you only mean to meet her parents just to know them. But it is a good way to tell.