The only way it would be confirmed 100% is if Paul decides to speak about it (which is very unlikely). But there is a lot of interesting things that, when added up, make a compelling case.

Check out some of my previous posts:

https://www.tumblr.com/undying-love/747116547607314432?source=share (John seemingly hinting at it)

https://www.tumblr.com/i-am-the-oyster/745421073667031040?source=share (this post was made by @i-am-the-oyster. It's extremely compelling)

https://www.tumblr.com/undying-love/745845642958700544/is-arrow-through-me-about-john?source=share (Arrow Through Me is very likely about John)

https://www.tumblr.com/undying-love/744309254463324160/why-pauls-song-fuh-you-might-be-about-john?source=share (why Paul's song "Fuh You" is highly likely to be about John)

https://www.tumblr.com/undying-love/763072273237393408/they-always-make-it-sound-like-johns-ex-was-paul?source=share (why do they always talk like Paul was John's ex and not Cynthia?)

https://www.tumblr.com/undying-love/743060695141203968/quotes-to-think-about?source=share (analysis of some quotes)

https://www.tumblr.com/undying-love/745385646421606400/paul-being-very-secure-about-his-sexuality-andor?source=share (someone who is an "immovable heterosexual" doesn't talk like this, imo)

https://www.tumblr.com/undying-love/740604882160091136/in-these-four-months-pauls-attatchment-to-john?source=share (very interesting comments made by Francie)

https://www.tumblr.com/undying-love/741853393262346240/bizarre-poem-written-by-paul?source=share (weird poem which may be about him and John)

https://www.tumblr.com/undying-love/740518716575563776/what-does-pattie-know?source=share ("What does Pattie know?")

https://www.tumblr.com/undying-love/746737159003832320/everything-that-paul-and-johns-girlfriendswives?source=share (comments made by almost all of their wives/girlfriends)

https://www.tumblr.com/undying-love/745712172768919552/paul-turned-to-linda-and-one-or-two-friends-and?source=share (do you think this might be normal platonic friends behavior?)

https://www.tumblr.com/undying-love/744843356938682368/mpls-pauls-london-office-interior-style-is (Paul collecting John-related things)

https://www.tumblr.com/undying-love/745773134413791232/paul-being-normal-over-john-a-compilation?source=share (Paul's comments about John)

https://www.tumblr.com/undying-love/742882446559838208/beatles-book-authors-saying-totally-normal-things?source=share (even their biographers noticed it)

https://www.tumblr.com/undying-love/748696306813386752/love-goes-both-ways?source=share (the way the look at each other)

https://www.tumblr.com/undying-love/743961690413105152/beatles-insiders-comparing-john-and-pauls?source=share (every time insiders compared their relationship to a marriage)

https://www.tumblr.com/undying-love/756850117398659072/some-lyrics-they-wrote-about-each-other?source=share (very revealing song lyrics)

https://www.tumblr.com/undying-love/755299637557067776/paul-describing-john-a-compilation?source=share (Paul describing John's looks)

https://www.tumblr.com/undying-love/759175217991057408/john-and-paul-being-normal-about-each-other-a?source=share (John and Paul revealing comments about each other)

https://www.tumblr.com/undying-love/760524606194974720/i-would-like-too-see-a-compilation-of-all-the?source=share (them being jealous of each others girlfriends, wives and friends)

https://www.tumblr.com/undying-love/760724033259061248/pauls-grief-over-time-a-compilation?source=share (Paul still not being over John's death)

Yes. Here's the full quote:

"Around Apple, in [Yoko's] hearing, Paul would sometimes be called John’s Princess. She had also once heard a rehearsal tape with John’s voice calling out “Paul … Paul …’ in a strangely subservient, pleading way. "I knew there was something going on there,’ she remembers. “From his point of view, not from Paul’s. And he was so angry at Paul, I couldn’t help wondering what it was really about."

-From Philip Norman’s biography John Lennon: The Life

Hi! I apologize that the post is a little confusing to read. The original excerpt from the book is a little more long and I tried to only include the relevant parts, that's why it looks a bit disjointed. But here's is a summary of what the passage is saying (and yes, it really happened):

Howard Kaylan, the lead singer of The Turtles, had a encounter with the Beatles at a club. He is recalling an incident where John was under the table taking upskirt Polaroids of women, and Paul was under the table too, helping him by lighting the scene with a lighter. Jane Asher, Paul's girlfriend, is clearly angry and tries to leave. As she does, Paul yells after her "Jane! Jane! Aw, come on, baby. We’re just having a little fun.” But Jane ignored him and kept walking.

There was an incident that happened in 1973, where Yoko and John were arguing at a party, and John said to her, "I wish I was back with Paul! (You can find it in Peter Doggett's book You Never Give Me Your Money). Also May Pang, who dated John around that time, said that John would say all the time, "I wonder what Paul is doing", and that he definitely wanted to work with him again during the time he was with her. Some people who read John's diaries said that John was obssesed with Paul, and that he mentioned Paul as often as he mentioned Sean and Yoko. There's lots of evidence that Paul was always on John's mind. That never changed.

Q: Can I ask what part of the documentary bought the biggest smile to your face? Mine was you and Ringo playing piano together.

Paul: There were quite a few smiles: me and John goofing around on ‘Two of Us’ acting like ventriloquists and singing through our teeth; me and John goofing around on ‘Bathroom Window’ where we start singing Tuesday - “Hello Tuesday!” - that was nice. But the one that immediately comes to mind is John dancing - just seeing him dancing. It’s very cute, and he was actually a really good mover! (x)

Yes. Paul has said many times that he had seen and noticed John around town quite often. He even said that he "tried not to stare at him too hard" because he thought John could hit him. And no, I don't think their meeting at the fete was a coincidence because Ian James, a friend of Paul's, said that he helped Paul to learn to play the guitar and helped him prepare for his meeting with John. If this is true, then Paul went to the fete to meet John. Which is funny because Paul has said that he went to the fete only to "pick up a girl." 😅

Yes!

Source: https://web.archive.org/web/20101109052459/http://imaginepeace.com/archives/12671

I've seen 3 different sources of this:

Yoko said it to Philip Norman for his book "John Lennon: The Life"(2008):

The other source is the book "Apple to the Core" (1972), written by two journalists (Peter McCabe and Robert Schonfeld) who knew the Beatles personally:

And Francie Schwartz (who dated Paul briefly and was present at some Beatles studio sessions) wrote a 4-page article for Rolling Stone in 1969 that said this:

Hi!! I don't have any post about it. The only time I have heard someone speculate about Girl being about Paul was Rob Sheffield, in one episode of the podcast Another Kind of Mind. I think it's this one: https://www.podbean.com/media/share/pb-nfrh3-1277b09?utm_campaign=w_share_ep&utm_medium=dlink&utm_source=w_share

John's reaction to Paul's arrest

"When the news broke in mid-January in 1980 that Paul McCartney had been arrested in Japan for possession of marijuana, John was extremely angry. They say that their relationship was not the best at the time. However, what some biographies suggest about this incident is far from the truth. John was not at all happy about this. On the contrary, he went so far as to criticise Paul and became angry with him. I had rarely heard him speak in such a biting way."

"The constant appearance of Paul's name in the press began to unsettle John. "You don't think they're mistreating him, do you, Charles?" "Aside from the fact that they are holding him in a jail cell, I doubt that there is any mistreatment." I said. "That's good. Not that I really care, you understand, but I wouldn't want to think that they were abusing him in any way." "You keep telling me how much you don't care. I begin to wonder if it's true." "Of course I care! Not that I want to, but you can't know a person as long and as intimately as I've known Paul and not care. I'm pissed at him and have been for years, but that's my private war with the man. It has nothing to do with the way I feel about his having this happen to him." [...] Two days later Paul was released. The concert had been canceled and he and Linda left Japan immediately. John greeted the news with a great sigh of relief. "I'm glad that's over. I feel like I've been keeping a vigil for him. Not that I care, you understand."

Tune In: The Beatles: All These Years (by Mark Lewisohn)

"[Me and Stu] ended up good friends, but we'd had a few ding-dongs, partly out of jealousy for John's friendship. We all rather competed for John's friendship, and Stuart, being his mate from art school, had a lot of his time and we were jealous of that."

-Paul (Anthology)

That's almost definitely not true. For some reason Paul doesn't seem to want people to think that he went to the fete to see John. His friend Ian James, who helped Paul to learn to play the guitar, said that he helped him prepare for his meeting with John:

And even though the day was very hot, Paul:

Lol, no wonder he has to say he only went there because he wanted to pick up a girl. Also, Ivan Vaughan (who introduced Paul to John) was a mutual friend, so it's pretty unlikely that Paul wasn't dying for Ivan to introduce him to John, based on the way Paul describes seeing John before the fete:

"It's pretty clear now that we were a huge influence on each other. Readers might detect duelling emotions in my recollections of John; that's because my relationship with him was very mixed. Sometimes it was filled with great love and admiration, but other times not, especially around the time The Beatles were breaking up."

-Paul (The Lyrics, 2021)

"To this very day, it still is a complete mystery to me that it happened at all. Would John and I have met some other way, if Ivan and I hadn't gone to that fête? I'd actually gone along to try and pick up a girl. I'd seen John around - in the chip shop, on the bus, that sort of thing - and thought he looked quite cool, but would we have ever talked? I don't know."

-Paul (The Lyrics, 2021)

"While it's true that Linda is the person I'm addressing, it's also true that I'm dealing in fiction. Starting with myself, the characters who appear in my songs are imagined. I can't state that often enough. I know that in some quarters it's felt you can't write about gay people unless you're gay, or about Asian Americans unless you're an Asian American."

-Paul on Maybe I'm Amazed (The Lyrics, 2021)