Hi Craig,
There are a few good questions to ask and some pretty obvious things to look out for.
1. Ask when the 100k service was done and if they have reciepts for it.
2. All too often owners dont know whether it has or has not been done so a good indicator is to listen for a rattle, hum or rythmic knock from the timing cover on the front of the motor (sometimes a new timing belt can hum a bit if it has been over tightened).
3. Always drive the car so that you can feel how the shocks, brakes and steering feel... Usually if the steering feels heavy or very loose it can indicate that the HICAS (4 wheel steer) is on the way out or dead completely.
4. Skyline engines (especially) RB25's are very prone to dropping coilpacks, make sure that you listen carefully to the engine when it is hot for a miss - this could be a dead coilpack (they're not cheap to replace).
5. When driving the car, listen carefully for a squeal or high pitched whine when the engine comes on boost, the standard turbos tend to become tired and worn out if they have been boosted over standard pressure and they squeal when damaged.
6. The general condition of the cars interior is a good indicator of how it has been looked after, if the seats, dash, console, carpet etc are badly damaged and/or really dirty it usually indicates that the owner has not cared for the car very well at all.
7. The owners general knowledge of Skylines is usually a good indicator of whether they have a real interest and passion for the car... If they dont know what HICAS means, or what type of oil to use that usually means they arent really an enthusiast.
I think thats about it... I cant think of anything else at the moment
I hope this helps.