This question is best for the Brewing Equipment forum so I expect a mod will move it. But to answer the question to the best of my knowledge:
* The newest ones (~2007 onwards) have a tapered chrome plastic group surround and two different patterns on the drip tray. It also has an adjustable over-pressure valve (internally) and a non-replaceable element - whole boiler must be replaced if the element goes
* The ones before that back to about 2000 or thereabouts have a metal vertical group surround and a drip tray with just circles punched out. Some elements are replaceable, not sure when this change was made
* Pre-2000s (approx) have sikscreened "Rancilio Silvia" on the front of the unit instead of the silver on black Rancilio badge, and a domestic portafilter which has a straight fluted handle
* The really early ones apparently had a brown bakelite or something handle but I dont think youll see many of those around.