Pretty much only the build sheet ubder the seat (if there) or the equipment label on driver's side under the hood give the trim package code. That code is the best proof. Mine is missing both, but I found the order sheet in the glove box from when the original owner bought it in 77. I called him and he verified it and the changes that had been made to the truck over the years. So if you don't have the build sheet or underhood label, best bet is to try and track down original owner. I know that is alot harder to do and I got lucky, but there were no records because 76-79 Chryslers archives were closed.
Check out machopowerwagon.com or dodgeconnection.com lots of info on them. And find Warlock by Skip Gibbs. Good book has all the standard and optional equipment codes. Good luck and show us some picks!