In Part 2, Chapter 5, we hear a great deal about the the ways in which Winston and Julia are different in their thinking and in the way that the perceive things. One of those differences has to do with the way that Julia looks at the Party's propaganda. She is much more inclined to disbelieve everything that the Party says.
The example that you are asking for has to do with war. Winston mentions the war between Oceania and Eurasia. Julia says that she does not believe that the war even exists. Instead, she says, the government shoots rockets into its own cities to keep the people afraid.
Here is the passage that proves this:
Once when he happened in some connexion to mention the war against Eurasia, she startled him by saying casually that in her opinion the war was not happening. The rocket bombs which fell daily on London were probably fired by the Government of Oceania itself, 'just to keep people frightened'. This was an idea that had literally never occurred to him.