There is no way to test for chickens that I've ever heard.How can I test or check for that. It's my first ever silkie
You have to keep up that 400 i.u. Vit. E, and give scrambled eggs or a can of tuna with it so she gets a little extra selenium to help absorb it. If she'll eat some sunflower seeds, that's got high amts of both in them.
Remember, this can take weeks to get over.
If you have others, they should be getting vitamin water (Poultry Cell or something like that) at least weekly too just in case.