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

VIN, Fender tag, build sheet? VIN start with BP?

You may need your build sheet. 367 is the Formula S code on the build sheet.
@Alaskan_TA is an expert at this stuff.

Formula S cars will have the Commando 273, HP single exhaust, tachometer, HD suspension with a front sway bar, and Formula S badging.