well, then its just about what expression and identity works best for you. and again, these things arent objective - if youd had different experiences in your life, you may have found yourself a trans man, or nonbinary. but youre not wrong about being a woman now, and you wouldnt be wrong about being a man or nb in this alternate timeline.
i think people put a lot of unnecessary weight on 'how do i know???' - its all quite arbitrary in the end, if you have doubts just try something out, if you dont like it as much as something else just stick to something else. it doesnt need to be so existential (ofc i realise for ppl seeking medical transition these doubts are thrown at them by medical professionals and there is perhaps more need to be sure of your decision if doing smth irreversible, but the level of 'fraud-catching' scrutiny in medical transition is honestly ridiculous)
maybe their gender is a phase! maybe my love of comics is a phase. maybe your identity as a woman is a phase. we are constantly changing and growing, but we should respect who someone is in the moment. if that kid is saying theyre a man, theyre a man. if they change their mind tomorrow, theyre not a man tomorrow.
like, i dont ID as a trans guy anymore, but i still talk abt that period in my life as 'when i was a man' - in that time, that was most definitely my truth. im just a different person now, and thats okay.
also, if id had better access to medical transition, had lived with a supportive family, had gone to a supportive school, i might still be man! my decisions about who i am and my gender are heavily influenced by my environment and what i have access to. if more people had treated me like a guy as a teen, if id met less nonbinary people and cool lesbians, i totally coulda been a man forever. i dont think that somewhere deep inside me theres a concrete truth on whether im a guy or a girl or a both or a neither, its just where i end up, and a smidgen of choice.