Dumbledore found out by using the "homenum revelio" charm. non-verbally, to find out if there was someone there or not. >
In a chat with fans on the-leaky-cauldron.org, J. K. Rowling answered a fan's question of why Albus Dumbledore was sometimes able to see Harry under the Cloak of Invisibility, explaining that he used the Human-presence-revealing spell with non-verbal magic.
TL;DR - Harry Potters glasses were enchanted Dumbledore so that he could see everything Harry saw.
He finds out for sure immediately after Nagini's attack on Arthur Weasley. This night is significant in many ways. While Harry sleeps, he enters Nagini's mind.
Dumbledore does not know what will or had happened or whether or not a time loop is there. But he guessed two things. One fact he know for certain is that Buckbeak the Hippogrif, which was tethered properly by Hagrid had simply disappeared. He was amused at that time.
The Tales of Beedle the Bard first appeared as a fictional book in J. K. Rowling's 2007 Harry Potter and the Deathly Hallows, the seventh and final novel of the Harry Potter series. The book is bequeathed to Hermione Granger by Albus Dumbledore, former headmaster of Hogwarts School of Witchcraft and Wizardry.
Dumbledore is trying to ascertain whether Harry will be entirely focused on the tasks that Dumbledore has planned for him throughout the year. Being in love might cloud Harry's judgement, or cause him to split his time and energy between a partner and getting that memory from Slughorn.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
One is that Severus Snape is a master of Occlumency and therefore a difficult target for Voldemort to see through. The second is that Harry himself was not able to block Voldemort, and therefore the overall plan between Snape and Dumbledore was too critical to risk its discovery.
Dumbledore knew that destroying Voldemort's horcruxes was the only way to truly defeat him. He believed that Harry was the one person who was smart, courageous, and perhaps lucky enough to find and eliminate all the horcruxes.
Nothing about Harry being a Horcrux, or Nagini being a Horcrux, or Voldemort having Horcruxes at all. Snape did not know and Dumbledore tried to phrase everything the way it would not specify the kind of magic.
Dumbledore reveals how thoroughly he loves Harry. Harry is brave, and Dumbledore is proud as any father would be. More than that, Dumbledore succumbs to a parent's weakness; he cares more about Harry's happiness and wellbeing than the wizarding world he is trying to protect.
Linfred's eldest son, Hardwin, married a beautiful young witch by the name of Iolanthe Peverell, who came from the village of Godric's Hollow. She was the granddaughter of Ignotus Peverell. In the absence of male heirs, she, the eldest of her generation, had inherited her grandfather's invisibility cloak.
We sense that Harry's education in personal responsibility is all part of Dumbledore's grand plan in giving Harry the cloak, because after the dragon affair Dumbledore returns the cloak to Harry neatly folded.
In Rowling's Harry Potter series, Dumbledore demonstrated the ability to possess wandless magic on several occasions, most memorably when he cast Aresto Momentum without a wand to save Harry from falling to his death during a Quidditch match.
Image via Warner Bros. Similarly to Harry, Snape's patronus represents a person. In the memories he gives Harry before his death, Snape shows Dumbledore his patronus, using the form to prove his love of Lily Potter, Harry's mother.
“Dumbledore didn't want to lose his wand at that point and Draco disarmed him. So that meant that the wand gave Draco its allegiance, even though Draco never knew it, even though Draco never touched it. “From that moment on, that wand gave its allegiance to Draco, and it wouldn't work as well for anyone but Draco.”
He would always resent Harry for being James' son instead of his. Snape even said that he didn't want anyone to find out that he was protecting Lily's child, “especially Potter's son” (pg. 679 of Harry Potter and the Deathly Hallows) because of his hatred for James.
Draco Malfoy's mother Narcissa was cold, cunning and devoted to the Dark Lord. But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
Her sacrifice gave Harry the power of love, a protection spell sealed with the blood spilled by her death.
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.
At one point in the play, Harry's son Albus uses Polyjuice Potion to disguise himself as Ron. When he encounters Hermione, this means he has to kiss her to keep up the charade. Nothing wrong with that.
It was just a coincidence.
Voldemort is planning the escape of his key lieutenants from Azkhaban and is alternately happy and frustrated. Those strong emotions make Harry's scar tingle. He also feels a similar pain when getting changed after Quidditch in Chapter 18.
The couple came to trust the Weasleys very much, allowing their daughter to stay at their house over the holidays, prior to the school term, and to go to the Quidditch World Cup in 1994. The two families were brought even closer when Hermione married Ronald Weasley and had Rose and Hugo.