Rear speakers were an individual option. If the car had them it would have had a different in dash harness (it would have 2 large 6 wire connectors) and it would have had a rear speaker fader knob with a 6 wire connector and 2 pots on it. These are hard to come by, and the B body ones will not work (they have a shorter shaft).
If you do not have the original dash, harness or knob, the only way to tell would be to check the broadcast sheet. There is no way to tell from the fender tag.