One way is to look at the 5th digit (engine code) of the VIN, the lemans had a 350, the gto came with the 400 or 455.
H=350 (lemans)
T=400 (GTO)
Y=455 (GTO)
X=455 HO (GTO)
Request phs documentation on the vin, the Build Sheet Sales Code
is 334 for the GTO Option.