Unfortunately, I don't know.
Even more unfortunately, I don't know who else would know. The person who made that informative other thread hasn't been on the forum in almost a decade.
I have read that many white geese can be sexed by color when young. The obvious explanation: females have one...
I do not know of a genetic test, so I think you would be stuck with test-mating.
Are you trying to have a flock of pure whites, that never throws other colors? If so, white females are fine. Any white male that produces 8+ white daughters and no colored ones is probably fine too. The more white...