That's a difficult question, and I'm no expert.
However, I know most all guns, lightsabers, and plastic capes ahve been reproduced. Some of the obscure items, such as ewok hoods and Dulok weapons might not have been, I really don't know. I know there are also repro AT-AT chin cannons and the little rubber Death Star "Rope" piece.
Some repros are metal. From what I've heard, repros are more rigid (possible brittle), and sometimes thicker.
I have a few weapons that I assuem are repro, including some lightsabers (the tiny tips are quite rigid, and I recall the old ones bent quite easily becasue they are thin.) I think these would break if I bent them. I also have a black Lando General pistol that I believe to be repro. I don't have one I know is original, but the gray version that came with Jedi Luke is quite thin, and the barrel on mine is discolored from being bent. The black one I have is very thick in comparison.
I'm sure there are a few other repro vehicle parts. People now days can also print out new stickers, which could be a problem with R2-units, but great for customizing.
I noticed the other day that under a black light, the stickers on my known vintage R2s did not glow. However, on one R2 I got from a dealer, the sticker is in nice shape and does glow.
Has anyone else ever noticed this? Can anyone else with a black light check?