How to ID the rear axle?

The fender tag would tell the tale, as would the broadcast sheet if built after a certain year.
There are no ID numbers to certify.
You can narrow stuff down though.
Big bolt pattern in any A body built before 1973 is a tipoff unless you have an ultra rare 1968 HEMI Dart or Barracuda.
The 8 1/4" axle was also a 1973-76 ONLY axle. The 8 3/4" axle was NOT available after 1972, if you have a '74 Scamp with an 8 3/4" axle, it was put there sometime after the car left the assembly line.