It was Albus Dumbledore who had somehow worked out that Lord Voldemort had made Harry Potter into a horcrux.
As originally, in Harry Potter and the Chamber of Secrets Albus Dumbledore tells Harry while Harry was in his office that:
Voldemort had unintentionally transferred some of his powers to him after it was discovered that Harry could speak and understand parsel tongue.
Then in Harry Potter and the Half Blood Prince Harry was in his headmaster’s office again as Arthur Weasley was attacked by Nagini (Voldemort’s snake).
As Albus Dumbledore asks Harry how he saw it before he sends Harry and the Weasleys to Grimmald Place.
Harry then finds the time to tell his godfather as Harry told Sirius that he was worried that he was being possessed by Voldemort as Harry thought that he had bitten Arthur Weasley.
Harry also let Sirius know that the occlumency lessons with Professor Snape had ended and Sirius told Harry he must continue to do ucclumency as it was important that Harry close his mind.
Though maybe the reason why Harry couldn’t close his mind to Voldemort was because of the horcrux inside him?
However, in Harry Potter and the Deathly Hallows Harry sometimes saw flashes of what the Dark Lord was doing or feeling.
Harry also eventually discovered that he could close his mind to Voldemort if Harry chose to keep himself busy like when he was burying Dobby, he had closed his mind then.
I also think that Lord Voldemort wasn’t aware that he had turned Harry into a horcrux. As that is why Voldemort decided to use Nagini.
Albus Dumbledore also had told Harry that he had suspected that Nagini was a horcrux.
Then why he would think this is because he had also told Harry that Voldemort has been keeping Nagini close to him.
Albus also told Harry that it wasn’t wise to use living subjects that are capable of thinking for themselves, and he also told Professor Snape that: when he sees that Voldemort has Nagini enclosed in a magical cage that it’ll be the ideal time to tell Harry what Professor Snape had needed to tell him.
Then in Harry Potter and the Deathly Hallows Ron describes how being around the horcrux had made him feel which had made people think: that perhaps how the Dursleys had treated Harry was to some extent unnatural.
This had also made people think of other possibilities.
As people have thought that Professor Snape might’ve been affected by the horcrux inside Harry?
However, in Harry Potter and the Philosopher’s Stone Percy tells Harry that’s Professor Snape when Harry had asked him who is that teacher’s name?
Though Percy also lets Harry know that most of the students at Hogwarts didn’t particularly like Professor Snape.
So, I don’t think Professor Snape was affected by the horcrux that was in Harry.
Then another person that people thought might’ve been affected by the horcrux in Harry was his godfather Sirius Black and I think Sirius behavior wasn’t affected by the horcrux in Harry as his behavior was natural.
As I think people had to be thinking a certain way for horcruxes to have an effect. As maybe horcruxes can sense when it is in danger.
As Ron thought and worried that Hermione preferred Harry to him which eventually led to Ron walking out.
He then eventually returned. As he figured out how to use Dumbledore’s cigarette lighter and when Ron came back, he also saved Harry from drowning.
Also, with Harry’s encouragement he used the sword to destroy the locket. Though I think the horcrux in the locket was maybe trying to get Ron to kill Harry instead.
It had also been said that the centaurs in The Forbidden Forrest foretold Harry’s death.
However, Firenze said to Harry in Harry Potter and the Philosopher’s Stone that perhaps the stars have been read wrong. Also, Trelawney foretold in her prophecy that neither could live while the other survives and after much investigating:
Albus and Harry found out that Voldemort had asked Professor Horace Slughorn questions regarding a horcrux, and he specifically wanted to know whether a horcrux could be split?
Information on a horcrux was also a banned topic at Hogwarts and Horace Slughorn tried very hard to cover up what he said to Tom Riddle.
As Albus Dumbledore had tried to get Slughorn’s memory himself sometime, ago before he had set Harry the task to get it.
Then as Harry was successful, they found out that Tom Riddle had wanted to make seven horcruxes.
Also, his reason is probably because the number seven is thought by some to be a magical number.
Albus Dumbledore was also probably trying to work out how Harry could survive, and this is why Albus had left clues about the deathly hallows.
As after Voldemort had killed Harry’s parents and Harry survived and Voldemort disappeared Albus Dumbledore said to Severus that there are ways Voldemort can comeback.
Then another thing that I wondered about is why wasn’t the stone in Gaunt’s ring destroyed when Albus Dumbledore had probably used:
Griffindor’s sword to destroy it. As I am thinking that perhaps the stone survived because it was one of the three hallows. Also, could Albus Dumbledore speak and understand parsel tongue too. As he left Harry the first snitch that Harry caught which in the game, he actually almost swallowed.
Harry then realized that he caught this snitch that Albus had left in his Will in his mouth but when Harry put it to his mouth words appeared and said I open at the close.
Also, at the time Harry didn’t know about the Hallows.
Then after Harry, Ron, and Hermione decided to visit Luna Lovegood’s Father:
Harry had believed then that he had two hallows.
Though by the time Harry had used the stone he had all three hallows.
As he had Draco Malfoy’s wand and Draco successfully disarms Albus Dumbledore of the elder wand although Draco didn’t know that Albus had the elder wand.
Voldemort also knew that the elder wand wasn’t working for him which is why he killed Professor Severus Snape.
Harry then had to let Voldemort do the Avada Kadava curse on him to get rid of the horcrux inside him though as I said Voldemort wasn’t aware of the horcrux that was in Harry. I also think that there was a specific reason why Voldemort had to be the one that did the Avada Kadava on Harry.
As Harry had Voldemort’s horcrux inside him.