Roan test

Hi, I'm after some info on the roan test. There is a Connemara stallion in Queensland, Australia, that the owner is wondering if he may be roan. He is grey and most likely homozygous grey but has had foals that have a smattering of white hairs on their butts when they are born, they do however go grey quite quickly. He does have siblings in Ireland (not sure if full or half) that are known roans. Someone has told his breeder you can't test for roan but I was pretty sure you can. Where is this test done and is it just a hair test? Thanks.