It's hard to tell you exactly what to look for, but my advice is to find a stock R33 because any modifications generally won't fly well with RWC testers so you won't be able to get it registered and even if you did get a dodgy RWC (let's call it what it is), you'll face the headaches of getting pulled over all the time and defected if you intend on driving it on the roads.
Very hard to find a stock Skyline.
But usual checks definitely would be to check if the vehicle is registered, not stolen and has a good service history to begin with. Also open up the engine bay and look for signs of it getting repainted or restored following a major accident.