Recently I bought a pair of axles from a stage 1. These are supposed to be 3.5,s. Problem. How do I check them on pickup as I cant pull the centres and I dont know any external pointers. So by playing with some diffs at home I discovered this.
Jack up one wheel or if just a loose axle, restrain the other from rotating.
Mark a point on the wheel and diff flange.
Rotate wheel or flange counting the number of turns untill the marks are back where you started.
7 turns of the flange to 3 turns of the wheel and its 4.7 to 1 diff.
7 turns of the flange to 4 turns of the wheel and its 3.5 to 1 diff.

Keith