well not really, at least not visually:
early ones are smooth aluminium, some have ribs in them, some have "alfa romeo" on the front upper surface, some along the side upper surface, and at a guess some have different indentations where for instance the air tube goes over, whether injected or carb, early or late, some have a boss to attach one of the bolts for the air plenum hard tube, maybe some even have the dip stick upper mount hole on the side to fix this (as even the dip stick changes sides from early to late, doesn't it??), some might even have the 2 bolt holes in the front to tighten the cam cover down up front, and others i believe do not have these holes.
But generally speaking early smooth types might be all fairly similar, and later ribbed types might be similar (except of course the late 80's models, as jcslocum says, which will have the VVT attachment up front on the inlet cam side)
What I have never looked into is whether 1300, 1600, 1750 and 2000 engines have the same size - but at a guess i would say: yes?
There were a lot of changes in the valve covers over the years. In addition to the VVT mentioned by Jon, covers may or may not have front mounting ears and/or crankcase breathers. Tab type metal oil cap on center or screw type plastic cap off center. Finned or un-finned. Alfa logo across the front or logo longitudinal (to name a few).