During the test phase earlier today, only my PC account MR5 which I created somewhere between 2017-2018 was eligable. Never bought prime access or anything on it.
My main xbox account which I started warframe with in 2014 did not had the cross save function but I was able to link it to my PC account. I created a fresh PC account a couple days ago to maximize my chances, and it didn't had the cross save function either.
So I would say, if you going to check if you are in, keep an eye on your oldest PC account and refresh your account page every once in a while.
Who knows when the next silent test phase will be.