I don't know of any way to see all the evolutions before purchase when they are first released, but if you check out
THIS THREAD people do post the evolutions as they are discovered, and eventually they are added to the early organizing posts (gen 20 must be on page 3 or 4 or so? Maybe 5).
So you can look there to see, before purchasing.
Although you might have to wait a day or three before this new one is added.
Speaking of this new one, I am very excited to see how she turns out! Firn, is she actually slimmer? Is that just my imagination? For some reason she seems very thin to me, and I'm not sure how that effect is achieved, since I think you usually start with the same base?