Well that's why when the law is concerned, it usually defers identification to a trained professional (e.g. as determined by a doctor ).

Besides that, there are things you can determine with certainty: The presence of a substance in blood for example

A physical examination by a doctor, possibly augmented by imaging, is an excellent way to determine sex if previous observations and tests have been ambiguous. This is rarely a great difficulty, but when it is, it's how DSDs get diagnosed.