Det finns en paragraf i Half-Blood Prince där Dumbledore förklarar (eller snarare spekulerar) hur Riddle fick reda på om sina föräldrar och släktingar:
"All he had to go upon was the single name ‘Marvolo,’ which he knew from those who ran the orphanage had been his mother’s father’s name. Finally, after painstaking research through old books of Wizarding families, he discovered the existence of Slytherin’s surviving line. In the summer of his sixteenth year, he left the orphanage to which he returned annually and set off to find his Gaunt relatives"
-Harry Potter and the Half-Blood Prince
Även från en annan del av boken ser vi att fru Cole på barnhemmet visste att Tom var namngiven efter sin far och farfar (så hon kunde bara ha berättat för honom, jag tror inte att det var en hemlighet).
“Did she say anything before she died?” asked Dumbledore.
“Anything about the boy’s father, for instance?”
“Now, as it happens, she did,” said Mrs. Cole, who seemed to be rather enjoying herself now, with the gin in her hand and an eager audience for her story.
“I remember she said to me, ‘I hope he looks like his papa,’ and I don’t lie, she was right to hope it, because she was no beauty, and then she told me he was to be named Tom, for his father, and Marvolo, for her father —yes, I know, funny name, isn’t it? We wondered whether she came from a circus —and she said the boy’s surname was to be Riddle. And she died soon after that without another word.
“Well, we named him just as she’d said, it seemed so important to the poor girl, but no Tom nor Marvolo nor any kind of Riddle ever came looking for him, nor any family at all, so he stayed in the orphanage and he’s been here ever since.”
-Harry Potter and the Half-Blood Prince