I always noticed the difference in how his voice sounds between films even back to the days of videocassette.
As well as in how he had red lenses in one film Star Wars, a shiny helmet mask in Return of the Jedi etc.
Other folks have mentioned differences in lightsaber props for Vader in Star Wars and Empire Strikes Back but to be honest i never noticed. There were other suit differences as well between Star Wars, Empire and Jedi but i’m no expert on that.