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Possibly the build sheet that glued to the gas tank.However,over time the papers crumple and fall,fade off etc.Its not easy to even read that from under the car because its usually glued to the tank out of sight.
There also suppossed to be a axle code tagged on somewhere at the rear end/axle.Cant remember exactly what it was,Im sure the others will post later.Btw,does the car have an Oil cooler?It would be a round box above the oil filter with 2 hoses coming out of it.I think thats another way to tell if it has the Z51 package unless Im mistaken.