How can I tell if my 1979 Pontiac Firebird is a Trans Am or not?

The VIN will not tell you. Look under the back seat for the build sheet. It's usually held in place by the seat springs. If it survived it will have the option codes for the vehicle. You can also try Allied Vaughn for a copy of the factory to dealer invoice or build sheet. If 1979 is available you can order a copy from their website. GM Media Archives is another website with a lot of information. You can also reach out to The Pontiac Oakland Club as well. Hope that helps! Jim