Well the dash has a black-faced panel with round gauges---exclusive to the SS models. However, some people switch in a Monte Carlo dash, which is very very similar (but not quite).
But in reality, without a big sheet you can never be sure. The entire car can be faked. Even build sheets can be faked!
There's a lot more to an SS than emblems and badges, though:
Also it would have blacked-out grille, wheelwell moldings, black rear bumper insert panel, chrome 14X7 five spoke sport wheels, twin rectangular exhaust outlets, SS grille emblem, SS rear bumper emblem, SS emblems on a black steering wheel and column. Also SS 454 emblems on the front fenders, power disk front brakes.
Chevy dealers don't have a clue about old cars. it's not the place to even ask.
Unless you maybe want to hire some kind of expert show judge or Chevelle restorer who knows every little detail, then really there's no way you'll ever be sure.
The "expert" will only verify a hundred little details, and generally, if very detail is there, even the obscure ones--that doesn't "prove" anything, but circumstantial evidence is strong.
also without the original engine, the car is going to take a pretty big hit on value.
If the seller can't prove it to you, then it's probably a fake.