- What is the proof that this person is not what they claim of? (ie, what ideology of yours makes you convinced of this?). This ties into deeply philosophical questions - what makes you "you"? Do you have a conscience? A soul? Is your brain the source of your identity or is your loins? Does your loins control your brain and thus your existence?
But on a more direct reply - anthropology has made some significant discoveries about the history of people - including trans people. I would encourage you to look into that for more information - assuming you wish to learn more about the subject.
I would also encourage you to google (or bing) terms such as "trans people brains" where there's a lot of neurological research being done. First google hit i find is https://health.clevelandclinic.org/research-on-the-transgend...
There's lots of information out there, and more is being discovered every day so it's very useful to keep your eyes open on evidence - and not personal beliefs.