What’s the exact process to identify a real 66 Formula S Barracuda?

Yes, early '66 models didn't get the fish emblems. They still used the V's. The only parts that visually carried over from '65 however, were the chrome plate below the back glass, and the horn button, but there were some other less obvious ones that did too. Near as I can determine, it was around the first week of Oct 1965 when the changeover happened.