how do I tell what I have

I just bought a used roadrunner 8 3/4 rearend all I want is the gears.
I see 742 casing and I am told it is a 4:10 posi.
The old way was to turn the tire and count the revolutions of the drive shaft.
There are no tires and the brake drums feel like the brakes are up tight. any ideas would be appreciated since I just bought on good ol faith.

Don:prayer: