Only you get to decide your own gender identity. Research consistently shows that gender identity is an internal, personal experience that cannot be determined by external observers. No one else has the authority or right to tell you what your gender is, regardless of their opinions about your appearance, your body, or how you express yourself.
This principle is supported by major medical and psychological organisations worldwide. Guidelines from bodies such as the World Health Organisation and the American Psychological Association recognise that gender identity is self-determined and that individuals are the ultimate authority on their own gender experience. Evidence indicates that when people insist they know someone's gender better than the person themselves, they are overstepping fundamental boundaries of respect and human dignity.
People often ask how others can know someone's gender identity with certainty. The answer is straightforward: the respectful and accurate approach is simply to ask the person directly. They are the expert on their own experience, and their answer deserves to be accepted and respected. This approach acknowledges the basic principle that each person has the right to self-determination regarding their gender identity, free from external judgment or imposed definitions.