67 S-code questions

you can get a Marti Auto Works report on the car. Yes, there should be a tag on the door.

You can look under the rear seat and see if there are two points for exhaust hangers. The GTs had duals.

Marti report is about the only sure fire way though.

https://cybernetixs.com/secure/martiauto/vinreports.cfm

This wont prove it was a "real" GT or anything but look on the engine components and rear axle for date codes and part numbers (the aluminum tags are particulary desireable to collectors). That should tell you if they are original components or were swapped out of a '69 Galaxy wagon or some such.

Assuming it is real and original and in excellent shape a big block 1967 Mustang GT fastback can pull in a pretty decent price, even more so if it is loaded with options. Things like factory AC, convenience group, deluxe interior/exterior, tail light blackout panels, tilt wheel, etc. can really drive up the price.

As has been said though, pay the money for a Marti report, if you are selling it the cost will be recovered. Good luck!