Ah, I think I see where the confusion is.

The “positive” or “negative” identification is in relation to what the person claims. So if a person claims to be a woman, we can use science to determine either “yes this person is definitely a woman” or “maybe this person is a woman.” What we can’t do is say “no this person definitely isn’t a woman” because it’s possible there is some factor we haven’t identified or discovered yet which would validate their identity.

Edit to add: actually, I can think of ONE test to prove that somebody who says they’re a woman but isn’t: gender transition to the gender they claim to identify as. Cisgender people usually get severe gender dysphoria if they attempt gender transition. I would consider that proof positive that they aren’t the gender they claim to be. However, subjecting somebody to such an experiment without fully informing them if the risks and/or against their will is massively unethical which, imo, disqualifies it for the purposes of this conversation. But technically it’s an option.