You can't but you can tell if it could have been original or better put if it could not have been.
All comes down to physical characteristics since there are no engineering number for the full assembly unless the ink stamp is still visible on the side of the main/rear part of the case
Other things that can help
1- Pulley type size and number (dual or single)
2- Shape of the lug (for lack of a better term) where the adjusting arm attaches to the front part of the case.
3- Shape of the rear of the main housing/case and if AUTOLITE is cast into the back side around the center of the shaft area. Change from earlier round detail to teardrop took place somewhere around the middle of 67 production
There are others but things like the wire loom and retainers, attachment hardware could have been changed or swapped with ease
Some of these details are likely visible in some of the Unrestored Picture thread listings under C or A coded vehicles
Others may have some additional methods. You might also post a few picture of what you have
