What I mean is, eight might be too late, I think much better to give much younger children huge choice of dressing up clothes and see if any make cross gender choices you can then watch them for consistency and maybe suss out with the parents if they are ok at home. No need to ask the direct questions if little Chardonnay always puts boys clothes and seems a whole lot happier when she does.
I used to be in dungarees all the time when I was little, then trousers, I hated dresses. But I knew I was a girl. I just got confused about liking other boys or girls until I was old enough to realise I like both