“When we replace the question “Am I sure I’m trans?” with the question “Based on the evidence that is available, and what my thoughts, behaviours, past and feelings suggest, what is more likely: that I’m trans or that I’m cis?” -Natalie Reed in The Null HypotheCis
To go a bit deeper, if you’re looking for certainty of your gender identity in order to continue or move forward with transition (defined broadly) then you are creating an impossible situation for yourself. You will always have those “what if” thoughts. Identity is a subjective experience and therefore cannot be 100% objectively verified by hard sciences. Reed elaborates:
Maybe someday we’ll have brain scans that can analyze the parts of the brain that are “atypical” (or, as I’ve taken to saying, extraordinary) in gender variant individuals and thereby determine whether or not you have the neurological features that indicate a predisposition to transgenderism, the best it could do is determine a predisposition. Such a system could never ethically be used as a singular, definitive diagnostic tool, and given the subjectivity and self-determined nature of gender, there’d still be an abundance of “false positives” and “false negatives” (though even those concepts don’t make much sense).