Having just bought an origional, there are a couple of ways.
origionals are 27 x 41, the modern reprints are 27 x 40. the main things to look for are
style A, a hair on lukes belt - fake
style b, no gau logo on the bottom - fake
style c, a hair on leia's back - fake
rolled posters are like gold dust, you should see fold marks because in the 70's movie posters were folded before posting to the theaters. if it helps, the guy i bought mine from has some others he is selling (he is trying to fund a couple of new tk projects. I could hook you up?
Garreth