It’s really not that difficult, as science has a very definite definition. If you have a Y chromosome, you’re male. If you don’t, you’re female.
There are few *exceptions* to this, and they are unbelievably extremely rare. A human chimera is someone who was born as a twin, but for whatever reason, absorbs the second zygote and they manage to cobble together without killing themselves. This can result in someone actually having a mix of Y and non-Y chromosomal makeup.
Siamese (conjoined) twins almost always form from identical twins (as fraternal twins have separate amniotic sacs and everything), so they almost never have different sexes. I think there may have been 5 cases (at most) in history of such.
I didn’t know that about human chimeras. That seems to explain why those anomalies exist. I’ll research more. Thank you.