I honestly don't remember where it's at anymore. It was in the 90s and 2000s when I still cared about the science of these things. Doing searches on it now only reveals shit that uses CIS-Gender and other woke talking points and isn't reliable. What I can recall is the structure of the brain, the parts more developed in women (as opposed to the development in mens) were similarly developed in gay mens brains and even more developed in those who were trans (born men). Hormones in lesbians and gays were slightly higher, both T and PGT (female T), the hormonal balance was stronger in trans. Leading theory is that exposure to higher levels of hormones, mostly T, during pregnancy lead to higher rates of LGBT people. Maybe it was a sign of bad diet, birth defect, a bad gland, who knows, but it was a real, measurable difference with a spectrum of reliability enough that you could point to someone and be reasonably sure by their hormone levels alone and say "Da gay es hur!"