DML FAQ : Transmission and Gears : How do I know what size rear axle (gear) I have?
From Jon Steiger:
The most certain way to tell is to just look at the rearend. The 8.25 is an oval shape with 10 bolts, the 9.25 is a diamond shape with 12 bolts.
Hey, how about that! I just did a quick web search and found this rather useful page. That's a bit easier than trying to describe it in words
From Bill White:
There is a sticker in the glovebox that tells you what rear end you have.