I don’t have the books in front of me right now so I am sure I am forgetting tons of potential moments! Feel free to add more.
- when Harry and Dumbledore were alone with the Mirror of Erised. His answer didn’t have to be “woolen socks”.
- someone could have written bathroom graffiti of “Dumbldore is teh gay” and Harry could have asked him about that.
- Hagrid could have accidentally outed him.
- Madame Rosemerta could’ve made an appreciative comment of having at least one guy not flirting with her.
- Rita Skeeter could have brought it up in her biography on Dumbledore’s life. or at any time, when making Dumbledore sound senile.
- someone could definitely have made a crack about Dumbledore’s trust for Snape and snakes.
- Dumbledore could have danced with a man at the Yule Ball.
- he could have had a picture of his lover on his desk.
- Phineas Nigellus could’ve made a snarky comment about homosexuality.
- if it was well-known in the wizarding world, Ron could have said something and then Harry and Hermione could say, “what the fuck?” Ron could lord his knowledge over their heads.
- on his Chocolate Frog card he could have been listed as the first (or second or third) openly gay Headmaster at Hogwarts School of Witchcraft and Wizardry.
- or if that didn’t fit on his Chocolate Frog card, surely it could have been mentioned in Hogwarts: A History.
- Dumbledore’s sexuality could have been gossiped about after an Order of the Phoenix meeting. Fred and George could have overheard.
- Remus or Sirius might’ve outed Dumbledore in one of their talks with Harry.
- Dumbledore could’ve mentioned that he was gay.
For everyone who has been saying ~but it would be weird~, here are AT LEAST A FEW THINGS that could have happened, along with a few things that SHOULD have happened (at the VERY least, Hogwarts: A History). But seriously, Hagrid outing him in one of the last few books would have been COMPLETELY in-character (oh, I shouldn’t’ve said that), potentially amusing, easily downplayed and mostly-forgotten, AND ACTUALLY HAD HER WRITING ABOUT A GAY CHARACTER, NOT A CHARACTER WHO SHE SAID AFTER ALL WAS SAID AND DONE WAS GAY.
From memory, in the seventh book, aren’t there massive excerpts of the Reta Skeeter’s biography written?
Even if Dumbledore wasn’t really out, don’t tell me that Rita Skeeter wouldn’t have been able to find some evidence and then written all about it. Outing Dumbledore would have been EXTREMELY in her character.