Counting the teeth is the most accurate but another way to check the rear end ratio is to jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.
As for checking if it has a posi.... a big smokey burnout will tell you if one or two wheels are spinnin'
Actually, if you jack both tires of the ground and spin one tire by hand the other tire will spin backwards on a non-posi, the same direction for a posi.
