His book explains that everyone suspected him of being a boy when he was young, given his obvious temperamental differences from his older sisters.
“[A]s I would get older, it soon became clear I wasn’t interested in the same things other girls were interested in.”
“My parents say I wasn’t soft-acting like baby girls usually were. They’d already raised four of them, so they knew. I wasn’t affectionate, and I didn’t want to be kissed or held much. What I wanted was to run around, take things apart and figure out how they worked, and I didn’t care about getting hurt.”
He also discusses beating boys up to “show the boys I wasn’t to be made fun of.”
“I was comfortable being around boys. We played and hunted together. I had no problem fighting them. I understood their language and they understood mine. With girls, things were not exactly that way.”
He is the one who first confided in his mother that he felt he would not develop breasts or get a period after noticing that he wasn’t developing like his female peers.
It seems like he always knew, in one way or another.
But he knew for sure when he was first tested for “high testosterone,” because he admits that the exam was a “gender test” and they discussed XY chromosomes with him. But he states that he found out about his internal testicles when the rest of the world did: when an Australian newspaper published the leaked results of his gender test, just after he had won gold at the Berlin World Championships at age 18.