Dealers are cleverer than you on all fronts. Best option is to take along some friend who knows JDM cars.
Things you cannot check:
- If car is accidented
- If mileage meter is reversed
- If engine is damaged
Things you can check, but with experience:
- Car's economy/average
- Drive comfort
- Car's pick, especially in fully loaded condition (4 people, with AC)
- Car's tyre state
- A limited chance to get auction sheet verified from Japan
Rest is all hit and trial, or your luck. Your best shot is to find a friend who has experienced a JDM.