If the numbers are not available as Jon mentioned - This is a great write-up on 8 & 9 inch rear ends. Take a look... Kevinstang's Ford Nine Inch Differential Page +
And the ratio is the number calculated by how many times the input shaft turns divided by the number of times the wheels turn.
First you have to determine if it is a single track or trac-loc differential. If you jack the rear wheels off the ground and turn the driveshaft and Both wheels turn in the same direction, then it is a trac-loc (or posi). If only one wheel turns and/or the other turns backwards, then its a single track.
For a posi - mark the tire and turn the drive shaft until the wheel turns one full revolution. Keep a count on the turns of the input shaft and that number is your ratio. If you have a 3.73, for example, the input shaft will turn 3 times plus almost 3/4 of a turn while the wheels only turned once.
The single tracks still confuses me. I beleive you have to have some helpers and get both wheels to turn forward at the same time you are turning the input shaft. Otherwise the process is the same. Maybe someone will confirm/correct this idea.