Although many differentials look different the layout is common. Below pictures can represent various differentials.

 

EATON FRONT DRIVE

Locate the output shaft as indicated in the below picture and count the splines (notches) around the shaft.

*Not all Eaton differentials include the output shaft.

MACK FRONT DRIVE

Locate the output shaft as indicated in the below picture and count the splines (notches) around the shaft.