In my personal opinion, if a person has reached an age where they can logically think, it is up to them. At the base of every transition is a desire for happiness: to be happy in their own skin. So if a person is a "false positive", but still full heartedly believes that transitoning will allow them to feel better about themselves, more at peace with themsleves, and more confident and assured in their worth as a human being and an individual, then I wouldn't call that a "false positive". And thats why its called transition, and not "change" or "transformation". It doesn't happen in a day, and its a long process. A process that procedes slowly step by step, which at any point a "false positive" could pull out and resume life with no real consequences or irriversable changes. Its only after months of hormones that the changes start to have some long-lasting effects, but even most of those are reversible. Thats why HRT is truly the test of false positives or not. A transgender individual enters into HRT with the idea that it will alleviate dysphoria and increase overall mental health, even if only a little bit. If it only worsens it (which would be "loudly" clearly felt by an individual, hard to miss that is) then the individual would know that it might be other things.

Another "test" as you might call it is the test of time. If the individual seeks help for other issues, while fully being open to the possibility of being transgender as well, still desires transition and to be viewed and accepted as the gender they identify with; well that's the answer right there.

That's why they say every story is different, but kind of the same. Every transgender individual feels a calling to transition, to their identified gender, even if they don't understand it or don't have the capacity to interpret it yet.