Need advice on finding rear end ratio

Just purchased a '67 C10 truck with 250 c.i.d. inline-6 and 3-speed auto trans, but the previous owner is not sure of the differential gear ratio. Questions: (1) Is there a tab somewhere on the differential housing that shows the existing gear ratio, or... (2) can I just jack up the rear of the truck and turn one of the rear wheels by hand, while watching how many times the drive shaft turns for one revolution of the rear wheel. Keep in mind that I have an automatic transmission, so could I just put it in "neutral" so as to allow the drive shaft/rear wheels to turn? The wheels are now 15" in diameter, and IIRC, the original diameter might have been 16". Would this affect the ratio reading? Thanks, Smitty

Check this link http://www.hotrodders.com/forum/1967-c-10-stock-rear-end-gears-154368.html#/topics/154368?_k=gu0596