Check the obvious things like the shape of the car etc..
look for white smoke from the exhaust when it is booted.(sign of oil seals in the turbo going)
check the vin plate on the bulkhead and inside the car, make sure the colour code ties up with the car.
make sure the seatbelts have got the tags in the bottom of them which states when they were made, if they are not there the car could have been "rung".
make sure the interior details line up with the manufaturing spec for that year.
Don't know what they are for the GTS but there are plenty of owners on here.
look at the colour of the oil on the dipstick.
look for seat and pedal wear out of line with the mileage.
have a drive and feel the clutch bite point too high or low could be trouble..
get documented proof of claimed mods and bhp/torque claims if there are any, ask for reciepts etc..
go with your gut instinct.
when you have checked all that then give me a call and come and buy my GTR !!