Look at and in the distributor itself.

Duraspark II distributors will have a "cog wheel" inside with 8 teeth. And you will see the magnetic pickup assymbly, will also have wires going to it. Ignition Module is located on the fender, and is independant of any computer control. Will also have a vacuum advance on the side. Most common before 1984.

Duraspark III distributors, they have no guts. They are just a rotor inside and some alignment screws. No wires going to the distributor at all. No vacuum advance. Ignition Module is located on the fender, and is dependant on the EEC-III computer system for control. Also has the CP sensor. Most comon on V8s in california before 1984.

TFI-IV distributors, the rectangular ignition module is located directly on the outside and bolted to the distributor. Dependant on the EEC-IV computer for control. Most common on trucks 1984 and up.

hope this helps...