Ident GNX
The quickest giveaway is the rear suspension. This is the hardest to acquire. Then look for the ASCMclaren decal in the doorjam, then to the ASC on the emissions plate under the hood. If it has all three of these then it's most likely real, or a real good fake. Most people that clone don't bother with the little details like the door jam decal or the emissions plate. I have only ever seen one clone or recreation to have al of the details correct on a clone, and the owner never tries to pass it off as real.
This will get you most of the way there. If you are considering purchasing one, definetly send the vin to GNXregisty to confirm.