#TL;DR ginny weasley knew as soon as that man came in for a cuddle

[whinlatter]

In your opinion when did Ginny start drifting away from Dean? What was it that made her decide she didn’t want to be with him anymore? I know it can’t just be him helping her through the door because it’s such a small reason to break up with someone. Did she start to notice Harry noticing her? Or did the relationship run it’s course?

are you ready for an unhinged galaxy brain take from me

I actually think Ginny started to suspect something had changed in Harry's feelings for her not long after he himself realised, after the Gryffindor-Hufflepuff match, early November. The scraps of evidence for this headcanon are few and far between but you can nonetheless prise them from my cold dead hands. Even before Harry himself realised he had feelings for Ginny, he was asking to hang out with her (on the train, in Hogsmeade) in ways that definitely would have seemed new. Then after the match itself:

'Laughing, Harry broke free of the rest of the team and hugged Ginny, but let go very quickly. Avoiding her gaze, he clapped a cheering Ron on the back instead as, all enmity forgotten, the Gryffindor team left the pitch arm in arm, punching the air and waving to their supporters.'

Ginny is a very good reader of Harry's emotions and responses at this point in the series. She seems to clock that Harry has just given her this very weird, out of character hug - weird both in that it's clearly very physically awkward and brief, but also weird in that Harry has given her a hug at all, when he's never initiated a hug with any other character before, let alone her. Then, this chat at the after party, immediately after Harry has just ditched a group of admiring (and extremely willing) girls flirting with him:

'“It looks like he’s eating her face, doesn’t it?” said Ginny dispassionately. “But I suppose he’s got to refine his technique somehow. Good game, Harry.” She patted him on the arm; Harry felt a swooping sensation in his stomach, but then she walked off to help herself to more butterbeer. Crookshanks trotted after her, his yellow eyes fixed upon Arnold.'

This is the first time Ginny touches Harry in the series. It's innocuous enough to anyone watching - Chaser pats Captain on the arm after a game - but given how unusual it is for Ginny to touch Harry, so soon after their previous strange interaction where Harry initiated touch with Ginny for the first time, I think we can see this interaction as Ginny testing her theory. (Tbh I think Harry's response to her here, including the fact that he literally doesn't even speak in front her lol, would give her even more reason for suspicion.)

I basically think Dean and Ginny are doomed after this. Not because I think Ginny would be like great Harry likes me time to break up with Dean - I think she's got months of pranging out about it ahead of her. But I think the stage is set for Ginny starting to actively compare Dean to Harry, and finding him wanting. Between the more morsels of evidence she gets in the following weeks (taking Luna to the ball, the maggot incident), and how miserable Ginny seems at the prospect of going back to Dean in the New Year, I think there's good reason to suspect she had clocked that Harry might, at long last, be returning feelings for her. (I tried to talk a bit about Ginny's view of Dean here).

I know a lot of fics and general fanon has Hermione working Harry's crush out first and pointing it out to Ginny, but I actually really don't buy this! There's no real proof of Hermione noticing Harry's changing feelings for Ginny until really late on. On the topic of Slughorn's party, she tells him to "just invite someone", and doesn't seem to suspect anything after he lies that there's no-one he wants to invite. I genuinely don't think Hermione knew until mid-March:

“Yeah, well, there was no need for Ginny and Dean to split up over it,” said Harry, still trying to sound casual. “Or are they still together?” “Yes, they are — but why are you so interested?” asked Hermione, giving Harry a sharp look. “I just don’t want my Quidditch team messed up again!” he said hastily, but Hermione continued to look suspicious, and he was most relieved when a voice behind them called, “Harry!” giving him an excuse to turn his back on her.

Of course, after this point, Hermione is in full super sleuth mode. To Hermione, Ginny's argument with Dean suddenly makes a lot more sense now she's realised that a) Harry likes Ginny and b) Ginny has already worked out Harry likes her and so is sabotaging her relationship with Dean. After Ginny and Dean break up while Harry is on Felix Felicis (subtle from HJP), Hermione knows for sure what's going on, and is deliciously smug and unsubtle about it in ways that makes me think if she had known before this, she would have let on. It's true that Harry is oblivious, but it's also true that Hermione has never passed up a chance to say I told you so (she's just like me fr)