bassman97's method works and is much more accurate than you would think. It's easy to tell 1/4 rotation, so you can easily see the difference between 3.00, 3.20, 3.50, 3.73, 4.10, etc.. I prefer to do it from the wheel hub, and count the pinion rotations... makes it easier to see EXACTLY one full rotation of the wheel, but that only works if it's a posi. If it's an open diff, you have to do it from the pinion. Either way, no removal of the diff cover is required (although you might want to remove it anyways, to see if there's rust on the internals, metal shavings, or chipped teeth on the gears).
well i bought the rear end and i learned something good at the same time -ford gears have the number of teeth stamped into them! the guy was nice enough to put the axle housing across two jack stands and remove the rear cover and show me 41 ring and 11 pinion! i must say it was a positive experience buying from an individual that was knowledgeable and honest