I agree, Barry - great question because I think I'm headed down the "rear end" road soon.
My first instinct says, "Get out your ruler and measure it" but I'll take a look around and see if I can come up with something better.
BTW, BT, I really enjoy the scenarios you create with the questions. I often see myself in them.
OK, try this. First, just take a look at the axle tag if it exists. That should tell you everything (look for the L within the gear ratio to denote traction loc - what we used to call "posi-traction" or just "posi".
If the tag is missing, you would see the the 8" to be much more oval-shaped, whereas the 9" is more rounded and almost always has two dimples, when viewed from the back.
To determine if it has posi, open them up and look at the spline covers. The posi will have recessed openings for 5/8" bolts holding the ring gear, and the open (non-posi) will have 3/4" bolts with a smooth cover.
Am I in the ballpark here?