In Room 101, O'Brien, Mr. Charrington, and Julia are together while Winston is strapped down. They're in the corridor of the Ministry of Love also. It is unclear if she is really there, or if Winston is merely paranoid that she is involved. Observe:
In Part 3, chapter 1:
The pain in his belly; a piece of bread; the blood and the screaming; O'Brien ; Julia; the razor blade.
In Part 3, chapter 2:
He was relating the entire history of his life to an audience who knew it already. With him were the guards, the other questioners, the men in white coats, O'Brien, Julia, Mr Charrington, all rolling down the corridor together and shouting with laughter. Some dreadful thing which had lain embedded in the future had somehow been skipped over and had not happened. Everything was all right, there was no more pain, the last detail of his life was laid bare, understood, forgiven.
1984 can be read both ways, with Julia as a spy or not. I think we all read it the first time that she isn't a spy. Our Romantic notions want us to believe that she loves Winston, or at least enjoys their pseudo-marriage together.
But, if you look at the sum total of evidence, it is probably naive on our part to think that she is innocent. So says Enotes:
1. Julia was outwardly an active member of the Party. She was in the Junior Anti Sex League, she spent countless hours in the Community Center.
2. In Part One when he sees her outside Charrington's shop, she might have followed him there as part of her spying.
3. She knew how to "travel" so they wouldn't get caught, for example coming two separate ways and going home two entirely different ways- not common knowledge to Party members and Winston is impressed by her knowledge and planning.
4. She already knew his name before she met him, even though they had never really crossed paths or spoken before.
5. At their first meeting she gave him a piece of authentic chocolate, not Victory chocolate, which she says she got on the "black market".
6. She gets several items from the "black market" for almost all of their meetings, coffee, chocolate, jam, sugar.
7. When she was young she was a Troop leader in the Spies.
8. She admits to total promiscuity with Party members, although no Inner Party members she does confess to having hundreds of sexual partners (possibly people she has turned in before if she's a spy).
9. He talks to her about his memories of changing records and she asks him questions, particularly if he was friends with Rutherford, Jones, and Aaronson.
Just as there are three friends (Rutherford, Jones, and Aaronson) and three world states (Oceania, Eurasia, Eastasia), so too are there three spies in the antique shop and room 101 (O'Brien, Mr. Charrington, and Julia).
It seems as if there might have been cooperation between O'Brien and Charrington. Charrington presented the apartment as if there was no telescreen in it, but it turns out that there was. If he knew it was there (which it would be likely that he did... it was his place) then he betrayed Winston. O'Brien obviously lied about being a part of Goldstein's group of dissenters to the party and set Winston up to get caught.
I do not think Julia was involved. I believe this because in the third book we find that she was tortured too.
As stated before, I believe the junk shop owner has involvement in Winston getting turned in.