of course you can wind them back - they are a computer at the end of the day. But some Jap cars are genuinely low km, as basically everyone there uses the mass transit system and might have only used the car on weekends. The cost of tolls, parking and driving in general over there are absolutely ridiculous - so lots of low km cars makes sense.
GTRs are very easy to spot ungenuine km, as all you have to do is look at the paint on the intake plenum - if it is still mickey mouse black, then it has under 100,000km - if it is all paint stripped and looks white in patches then it is easily over 100k.
Steering wheels, gear knobs, handbrake boots, seats and seat belts are easy indicators of km travelled, but can of course be changed too.