Who told Toko that she was adopted? In the 4th season of Maria Watches Over Us, it's revealed that Toko was adopted. In the final episode, when she and Yumi went on the half-day date reward which Toko won for finding Yumi's Card, Toko reveals that her real parents died in a car crash which she was the sole survivor of, and she was later adopted by the daughter of the head doctor of the hospital, who happened to be a classmate and friend of Toko's biological mother.

Toko tells Yumi that she had suspected for a while that she was adopted from overhearing "cruel adults". However, it has only been confirmed recently in a couple of episodes earlier. During the scene where Suguru Kashiwagi is driving Toko, he gets angry that Toko has found out and starts questioning her who opened their mouth, naming 3 women (I can't remember their names, but I believe they were the ones who tried to shame Yumi during the 3rd OVA Season). However, Toko doesn't answer, being more scared of Suguru's aggressive speeding.

So I am wondering, is it known who told Toko she was adopted, or did she found out herself?
Nobody ever told Toko directly that she was adopted. As you mentioned, she overheard her relatives speaking about it, and she confirmed it by looking it up in a registry.

Quoting a line from Toko's article on wikia:


  She found out the truth later after listening in to some of her relatives' conversations, and confirming it in a registry


So Suguru just got angry and there was no specific person who revealed the secret