The VIN will tell you nothing as far as if it is a true Z/28. The build sheet, if still above the fuel tank, will. If the engine is original it would have the partial VIN stamped on it. Any other documantation from the dealer or previous owners. Everything else is just looking for clues. Is the paint in the door jambs original? Look for the 15"x7" tire sticker. Also Jerry MacNeish may be able to do a verification for you.