galactiboy
OT Preferist
Off the top of my head:
ANH: over armor cape, red eye lenses, ringed gauntlets, chest box lights and details, armor should be a bit dustier, helmet and faceplate has two tone paint scheme, one black tusk on the facemask, black cape chain (not always seen).
ESB/ROTJ: under armor cape, black lenses, vertical lined gauntlets, more detailed chestbox, armor has a shine to it and the silver and black scheme, no two tone paint scheme, all black, silver tusks, silver cape chain.
One of our Vader experts has a site they will most likely post again.
Don't forget ANH also has the widows peak... thats a pretty distinctive feature of that helmet. Also I think the lights on the chest and sides are different colors, although I might be making that up.