Heres a few pictures I have taken of both my 8.5", and a 7.5" rear end out of a 2wd S-series.
Here are the differences, and easiest ways to tell between the 2.
As you should know, there are 2 main visual differences between the two.
First, here is a 7.5" rear end. Notice how the axle tubes are straight, and how the rear end cover is more square.
Close up of the axle tube.
Now, heres a picture of the 8.5". Notice how the cover is almost completely round, and the bugle in the axle tube just before it goes into the pumpkin.
Close up of the axle tube. Notice how the tube bulges right before it goes into the pumpkin.
A shot comparing the 8.5" cover (left) and the 7.5" cover(right).
Now, as I've said, most usually you'll have to have your driveshaft shortened going from a 7.5" to the 8.5" This picture should fully explain why. Looking at the flat surface where the cover would mount, notice how much taller the 8.5" is.
Hopefully that should explain the visual differences between the 7.5 and the 8.5", and how to identify one.