How do you know if a 66 barracuda is a formula s or not?

I found a 66 barracuda with a commando in it but do not know how to identify if it is a formula s car or not. Is it in the serial number or the fender tag? I have been visiting this site for a while and am looking for a 66 formula s with a automatic for about 4 years, now that prices are returning to a level that hobbies can enjoy cars again I hope to pull the trigger.