Hi,
I live in Tokyo and have an 85 3.2 Carrera.
I don't think the general rule in spotting a nice 911 here is any different from the US -- after 20+ years, they've gone through a lot and inevitably some become junks and some receive TLC.
Some differences would be:
* Speedometer in Km
* Manuals in Japanese
* Japanese Radio (difference frequency ranges for AM/FM)
... really not that much.
As mentioned, the "shaken" system, which mandates bi-annual government inspection of all cars, tend to keep the cars in good, stock conditions.
Also many have low mileage as they are typically owned as a weekend driver and the country itself is pretty small to begin with.
I guess the biggest obstacle would be you cannot check the car yourself, unless traveling over the Pacific.
