HOW DO I NO IF MY CHEVY REAREND IS A 8.2 OR8.5?

What vehicle are we talking about?

If you're looking at the older rear ends, you can generally tell from the diff cover. Most 8.5's use a fairly round cover with tiny little square "horns" that protrude at the bottom on either side. Some use the cover that looks like the 8.2, though, which is fairly irregular looking. The 8.2 cover has a diagonal "dent" across the surface.

Look at the illustrations on the nova forum below - you'll see what I mean.

If you can get code numbers off the rear (I haven't done this in a long time, but I think its on top of the passenger's side axle tube - you may need to use a scotch-brite pad or wool to try to get to this number, though. I know the information I'm giving you seems nova-specific, but that's only because those guys are always trying to poach the better rear ends - they're the ones always stuck with the 8.2's!

If it is a V8 then it should be 8.5. But we need model and year, and can easily look it up. Caprice/Impala most common.