You don't need to know any deciding factors to observe the decision. Sexual orientation, at least if we are talking about mostly-heterosexual vs mostly-homosex

You don't need to know any deciding factors to observe the decision. Sexual orientation, at least if we are talking about mostly-heterosexual vs mostly-homosexual as a trait is first observable around puberty, and doesn't seem to change, ever. So puberty is an upper bound for the determination having been made. And I really can't imagine that it would be this hard to figure out what factors "turn someone gay" if the decision is made closer to puberty. If it were already determined at birth, much like gender, it would fit with what we observe about homosexuality not being a choice, not being "curable" or "preventable". I'm not a scientist with deep knowledge in this topic, but from what I understand, there is no evidence pointing to the determination happening after birth.