I found myself thinking about this. The book on the surface would have you believe that Julia was in love with Winston.
However there is perhaps more evidence to show that she was working for the party.
The most common belief around 1984 is that Mr Charrinton handed over Winston
and Julia to the thought police and I would like to give some evidence that Julia and Mr Charrington were actually both working together, without using the hallucination that Winston had in prison I think I can demonstrate this.
1. Julia who by all accounts including Winston's should not have been attracted to Winston, told Winston that she loved him before they ever spoke. He is 10-15 years older than Julia, and he is unattractive, with fake teeth and varicose veins.
2. Julia somehow knew Winston was against the party when Winston thought Julia was a member of the thought police. If Winston had been loyal to the party Julia's message would have resulted in her death. Surely she was aware of this yet she made the first move.
3. Julia encouraged Winston to speak to O'Brien, Julia seemed only to want to live comfortably but was willing to risk everything they had in order for Winston to speak to O'Brian.
4. Julia had access to things that Winston and other outer party members couldn't get and it was seemingly easy for her to acquire them, such as real coffee, sugar, tea etc. Small comforts reserved for the inner party members.
5. Julia always seems uninterested in fighting the party, she only seems interested in her and Winston's relationship. Winston describes her as "a rebel from the waist down." In the book he doesn't understand why she finds this so funny. Does that mean she was a party member from the waist up?
6. Julia says she has had relationships with lots of party members but all the party members she had relationships with had been vapourised or not spoken of yet she remains. She had her first relationship with a party member at 16, as a member of the anti-sex league.
7. Julia is more skilled than anyone else in the book at evading the party, she seems to understand intricate details of their methods of surveillance. She gives Winston advice on how to avoid detection which never occurred to Winston.
8. O'Brian makes contact with Winston in a very similar way that Julia does, in O'Brien's office, O'Brian uses the party term "comrade" to refer to Julia twice and he seems uninterested in her, except for a bow of acknowledgement, or respect. All of O'Brians attention is focussed on Winston.
Anyway that's off the top of my head, there's a little bit more but I think I got the main points.
You want to believe that Julia just loved Winston but on reflection this probably wasn't the case. 1984 is one of those books that wrote itself as much as Orwell wrote it. It's fun to speculate about the meaning.
I hope he came to the truth in the end.
https://www.spectator.co.uk/article/orwell-vs-god/