When we used to scour junk yards, we had a habit of checking every A body and F body for the gears. If it was less than 3 turns, we walked. If it was less than 3-1/4 (but more than 3) we knew it was 3.23. If it was over 3-1/4 but less than 3-1/2, it was 3.36. If it was 3-1/2 we knew th gears were 3.55 and we would pull the cover to see if it was a 2- or 4- pinion. If it was a 4, we'd pull the rear end. If it was 3-3/4 (or more), we'd pull the rear and worry about the type later.

Twice we found 10-bolt posi 3.55 rear ends in A-body cars in the junk yard that had been skipped over. The two-turns-count-the-drive-shaft-rev's method never failed.