This is an interesting question. 1984 is told from Winston's point of view so that we never really get inside Julia's head. How do we even know what love means to her? What we know is that she is a down-to-earth, pragmatic person who wants to have a good time. She doesn't care about politics and doesn't share Winston's passionate interest in history. So it would be easy for to her to see her affair with Winston as just another in a long line of casual flings that perk up her life. We would also be suspicious of her love if we heard about it secondhand from Winston: after it, it would be what he most wanted to believe--and Julia would have an interest in having him believe it as well.
Yet the novel offers evidence that Julia is indeed in love with Winston. First, when the two of them go to O'Brien's apartment to talk about rebelling, O'Brien asks them a series of questions about what they are willing to do to overthrow the state. They easily say yes to a list of questions that include murder and throwing acid in a child's face. However, when it comes to being torn apart from Winston, Julia's response suddenly changes:
‘You are prepared, the two of you, to separate and never see one another again?’
‘No!’ broke in Julia.
The vehemence of her refusal to be separated from Winston suggests that she really is in love with him. At the end of the book, another strong hint emerges that Julia was once in love with Winston. She alludes to her torture when they run into each other and says that when faced with her ultimate fear she wanted to betray Winston:
You WANT it to happen to the other person. You don’t give a damn what they suffer. All you care about is yourself.’
‘All you care about is yourself,’ he echoed.
‘And after that, you don’t feel the same towards the other person any longer.’
‘No,’ he said, ‘you don’t feel the same.’
Her words, "you don't feel the same" indicate that she was once in love, but after her betrayal of Winston, a deep shock to her, she can't be anymore. The Party has broken that most intimate of bonds, the love they shared, and that has helped to destroy them both.