You can tell them apart by the diff cover. It is hard to explain, and I wish I had a picture to show you the difference. The 7.5" has an overall shape of a square with round corners, whereas the 8.5", the raised section almost resembles a football. You probably won't understand all if this unless you see them side by side. If I can find a picture tomarrow, I will post so you can see the difference.
P.S. they are both a 10-bolt.
"I used to have a handle on life, but then I used it as a plunger and broke it" -The Wave
ASE Master Tech + L1
'87 Trans Am-K&N,Cold air induction, SSM SFC, Flowmaster, 16" GTA rims.
'97 Bonneville SSE About Me