There are a lot of things to look out for and I think most enthusiast will give you advice that will walk along the same lines...
Year, Model and Specifications
The most obvious one is the Year and Spec of the car itself. Once you've found yourself a car, it wouldn't hurt to do a little history look-up on the dealer themselves. A lot of dealers these days claim their cars are of later models and of higher spec. I've seen a lot of GTR's who claim to be of 2002 but are not. Claimed to be of V-Spec or V-Spec II or even V-Spec II Nur but are most definitely not. Everyone wans to get what they pay for so be careful and be on the look out.
Usually, you can more or less tell what year the car was manufactured by looking at the sashes of the seat belts as there would be an indication there. But unfortunately, shoddy car dealers now cut those sashes off as consumers are becoming more aware.
The GTR34 was released in January 1999 in a choice of either standard or V-Spec, and to celebrate the new models arrival there was also a 300-car limited edition that wore a coat of Midnight Purple II paint. These models had orange front and side turn signals. The cabin had grey interior with red stitching. The center console was gold in color and the back light of A/C display is greenish. The VIN plates located in the engine bay, were usually made in blue in color and the engine bay itself is left black and unpainted. Also note the color of the writing of the fuse box cover is of white in color.
Some of these earlier models also come with a DVD Navigation unit located under the center console air condition vents.
Roughly, V-Spec models has everything of the standard Skyline R34 and came with
Modified active LSD controlled in conjunction by a viscous coupling
Stiffer suspension
Special front and rear Venturi style diffusers (rear is made from carbon)
Attesa E-TS PRO (can redirect up to 50% of the torque to the front wheels within 1/1000th of a second.)
Combination meter with compressed scale below 3.000rpm
Brake air duct
In August 2000, and additional spec was added to the lineup which was the V-Spec II. This brought on a few slight changes made to the cars manufactured. The now later models came in two new colors which is the White Pearl and Sparkling Silver. The N1-spec rear calipers were also used along with clear front and side turn signal/indicators. To differentiate the cabin of the newer models, the interior color had now been changed to black and the center console has been changed to iridium color with a white-ish back light. Aluminum sport pedals have been added to the interior as well. The writing on the fuse box cover has been changed to a yellow/orange color. The VIN plates of the later models were now silver in color too if I am not mistaken.
V-Spec II models has everything of the V-Spec models plus a carbon fiber bonnet with NACA duct.
In May 2001, the M-Spec was released. This model was based on the V-Spec II but it was aimed to be a luxury version of the car. It came with softer suspension known as Ripple Control Dampers which reacted more smoothly with the irregularities in the roads. The rear stabilizer was also softened and the front seats came with heaters. It also came with a complete leather interior and Nissan only produced 50 units of M-Spec a month. The M-Spec came in a special paint finish called Champagne Gold.
There was also an extremely rare special N1 spec. According to Nissan’s own records, only 45 were produced over the R34’s five-year model spread. These cars were stripped down specials. There were no air conditioning, rear wash-wiper, rear light and in-car audio. The N1 came with a hand built motor from Nissan’s race engine division. This engine used a stronger block, a fully balanced bottom end along with forged pistons, new con rods, oil and water pumps, different exhaust and steel-internal turbos. An even bigger oil cooler was fitted and bigger rear brakes too.
In February 2002, Nissan had announced the final model of the GTR34’s, which were the Nur editions of the V-Spec II and M-Spec. These models both used an N1 engine from the homologation special. The Nur has a 300km/h meter cluster (speedos). Nissan used the same speedos during the test in Nurburgring. The Nur also comes with a badge on the bumper which is a proper 3D cast badge and not like the stickers on the V-Spec and V-Spec II's. Nur's have a gold colored VIN plate instead of the standard blue or silver VIN plate. The engine cam covers are also covered in gold. The M-Spec edition of the Nur still had leather seats but the seat foam was altered to give a better ride in conjunction with the Ripple Control Dampers. Only 1000 Nur editions were made – 250 M-Specs and 750 V-Spec IIs and they were all sold on the day of release.
In December 2004, NISMO issued a press release of none other than the Z-tune. Only 20 of these will be made and only carefully selected donor cars will be used to make this "finest compilation of the GT-R". This car was trimmed in Alcantara and a redesigned steering wheel. The motor was swapped to a Z2 engine which took the capacity up to 2.8-liters and used components tried and tested in every form of motorsport that the Skyline has raced in.