You can sometimes pull back the rubber seal around the filler neck and peek down on the top of the tank and see if there is a sheet. You can also look from underneath with a flashlight and a mirror, after removing the spare tire holder and top cover.

There are some subtle differences in the original tanks compared to the reproductions, and some repros are much better than others. The way the extra metal at the seams of the tank is bent, and how much there is is different...the repros usually have more excess than the original. There is also a difference in how the tubes on the top of the tank, for the vents, are soldered. It's hard to tell without having them side by side.
Some repros will NOT have O.L. Anderson(the OE supplier of the tanks) embossed on them, either, which is a sure sign.
hth


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Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

If you have a small digital camera that you can get in behind the the rubber seal with, you may be able to snap some surveillance shots and see what's up top  

I recently found mine by pulling back the rubber seal around the filler neck, as Joel suggested.

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