The most positive way to tell is to remove the cover and look at the ring gear bolts. The 8.2 has 3/8 bolts with 9/16 hex heads. The 8.5 has either 11/16 or 3/4 hex on the bolts (I forget which). An easy outside i.d. would be the crease accross the cover of the 8.2 (like a 12 bolt). But these covers interchange with the 8.5.
8.5's came in '73 and up vehicles and stayed mostly the same until the late 80's when they switched from 28 spline axles to 30 spline. The pinions changed at this time as well.