After seeing the original trilogy Star Wars films on TV, I knew what to look for in terms of probable garbage mattes. Next time I saw the films in the theater, sure enough, I could spot them. That said, the first time I actually noticed them was on TV, even after seeing the movies in theaters many times.
Well yeah I mean realistically, once you know where to look you can find them.
With the successive tweaks (1997, 2004/2011) that were done to the SW trilogy the noticeability of the garbage mattes have gotten better.
The versions on cable (primarily on SpikeTV) now are the 2004 versions.