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By pulling the cover, you can spin the axel (very easy to do) around until you can read the stamp on the edge of the ring gear. You'll most likely see a number like 327, 355, 373,410, 411, or 456. The higher the number, the lower the gear and probably more desirable to someone looking to buy a rear end. The upgraded/lower gear isn't very expensive in itself, but it's not easy to swap them, so people will usually pay more if the work has already been done. Also, a look inside will tell us if it has a stock-style or aftermarket carrier. It will be a little more difficult to find out if a stock-style 31-spline carrier is in it. The passenger car/mustang 8.8 rear ends had 28-spline carriers/axles. A popular upgrade is to pull a 31-spline stock carrier out of an explorer rear end and buy new aftermarket forged 31-spline axles to match. I don't have a lot of experience with this upgrade, but I figure it may be a little more difficult to check unless someone pulled an axle out. That's a good 30-minute job to take one out, check it, and put it back in.
But I can tell you that you need to pull the cover to check the ring gear ratio. If you sell the rear end, that's the first question someone will ask. Hope this helps!