Your best proof is a numbers matching engine and the fire wall/heater box will be setup for a big block. All big blocks are SS's. Its still hard to prove a X body car was a SS.

The bad parts is there is no way to be 100% sure it was an SS from the factory, everything is available to make an original inline 6 into a SS. Even down to the aged counterfeit documents proving its an SS. You just need the correct casting number for the block and it can be "re-stamped" even cowl tags can be made.

 

Unfortunately, everything you say about cloning an SS above is true. I've seen the documents you've referred to for sale on Ebay before and wouldn't be able to tell they were fake. I knew the heater box and firewall differences applied to the 67 and 68 models, but I wasn't sure it included the 69 too. I guess it would make sense that it would be the same.
As for determining the build date, I would think the Fisher body number could help to figure out if date is the fall of 68 or fall of 69.

 

I'm not up on that but as Gorn posted below there's really no way to confirm it's a real deal short of one owner documentation from new.

 

Since the trim tag Fisher body number was reset to 100000 in August 1969, your VIN will be more helpful for determining whether you have an early or late model. 1969 VIN's started at 500001, and there were 243,085 1969's built. Also, the Fisher body number on the trim tag is unrelated to the VIN.