It's too early to tell. I've seen a few white hairs, but not enough to be able to say for sure. So far, she's just got a wisp of a crescent moon on her forehead.
Yeah - I don't know a thing about roans... My experience is with Arabs, who you can tell are going to grey because you see it on their face early on. Your roans don't start there. She may be like your other one, who looks completely different every year!