Everything about the axle is different. From the center support to the wheel bearings including the 3 pieces of the axle.
I do not know for sure if you can tell them apart by just looking at it but assume you should be able to. Most find out they have a heavy duty axle when they try replacing the wheel bearings and the parts they ordered out of the book do not fit.
The solid non-adjustable axle is the industrial version not the heavy duty.
If you can not get a answer here research Tony Jacobs on this board. He has several post with his email and/or phone # listed. He would know the answer you are looking for.