The only way you are going to be able to be 100% certain of the fabric is if you get the info from the original manufacturer/designer. Like I said before, I recall that it is organza over satin from a description I saw last year after the dress
first came out. Thats to the best of my recollection, but maybe someone else can give you a more concrete/definitive answer.
Any dress company worth their salt is going to be able to look at the gown and tell you what it needs to be made of any way...I know dressily will give you suggestions based on what it looks like. They seem happy to accept very specific requests
from brides, but I don't think they count on the brides to know what materials the gown needs.
But again, I believe it's organza over satin.