The simplest way "for me" is "if" the strengthening ribs are "glassed over" ( rough glass finish over the ribs) and do not look like the sheet metal frame , you "could" have an original. You must be aware that there are very nice repops of port hole hoods out there. IF the support frame is smooth fiberglass resembling the steel frame work and bonded to the outer hood, you DO have a repop. NONE of the original "all glass" hoods used a fiberglass copy of the original steel frame work. And you know how difficult it is to say "never" but in this case it applies , no question. In "my" personal viewing experience , the '65 progression was porthole ( past 272 which had one for sure) to all glass no porthole , to the last few glass with steel frame '66 carry overs used the same glass/steel version. "Around" Hertz production , Pidgeon Manufacturing ( a Ford division) made the all steel version intermittently used due to paint issues followed by a glass / steel hood with a different scoop opening than the first version , "possibly due to a "source" change. The sides of the scoop opening are "rounded( as in cut out that way) versus the first version where the opening was "blended" without obvious cutting marks. "I" am 99.999% certain that none of the "6S" serial numbered cars were produced with an all glass hood "from SAI" You are welcome to proove me wrong on any of the above.
Randy