It doesn’t really work that way… if she was told “you’re trans” and wasn’t, she would experience the opposite type of gender dysphoria over time and would want to express masculine/boy identity in line with her birth gender and physical traits.
She’s young enough that it’s not infeasible she might change her mind. Ideally she has been to a licensed therapist and/or psychologist who verified she met the latest DSM diagnostic criteria for gender dysphoria (the only successful treatment for which involves transition), and they are helping the parent manage this process. They’ll be in no hurry for medical intervention since puberty would be 4-5 years away most likely.
That being said, if you meet enough trans people, it’s not uncommon to hear stories about “knowing since I was 4”. Oftentimes it’s accompanied by a regret that their feelings were suppressed and they had to go through the torments of puberty “in the wrong body” as it were.
My own story is different. I didn’t “know” until my thirties, even if I can see signs going back to childhood in retrospect. Apparently at three years old I told my mom “I’m a butterfly and I’m going to be a girl”. She never told me that, I found out after she died last year, looking through her old notes. She assumed I meant reincarnation. Ironic considering I have adopted a butterfly metaphor to express my transition to a woman.
Anyway, yes, an eight year old can absolutely tell adults they are a different gender than they were assigned at birth. Whether they literally say “I’m trans” is kinda beside the point.