The first thing to look for is any evidence of rear speaker(s). Check for wiring that would have run down the firewall on the passenger side, down the passenger side floor pan and into the trunk. It would have exited behind the rear seat back on the passenger side.
For reference pictures:
Scroll down to nearly the end of the page for Jim's survivor car resto:
A picture of the exit area on my custom wiring that mimics how the factory installed rear audio wiring:
A single blue wire or blue and black wire would indicate a right rear speaker (optional U80 rear speaker). This would have ONLY come with mono radios (am pushbutton and am/fm-non stereo).
A three conductor black/blue/brown would indicate stereo equipment (not necessarily FM though!!! i.e. AM combination 8 track player).
Okay, so let's say you don't find any evidence of rear speakers. You can rule out AM/FM-stereo, AM/8track and AM/FM/8 track as they ALL were installed with 4 speakers.
It also rules out optional right rear speaker with mono FM and am pushbutton radios.
Now, if it is down to AM/FM (non stereo) and AM radios, you could check under-hood and see if the purple blower motor power wire that comes from the harness is directly connected to the blower motor under the passenger fender.
AM/FM cars got a "noise filter" installed in-line with the power feed of the blower motor. It looks like a condenser found inside the old points distributors, only difference is, it has a mounting bracket on the side of it, and a wire coming out of each side.
Now, like I said, if a car has been "redone", all of the "evidence could be long gone" and the only true way to find out what the car came with is the build sheet.