Why does Julia find it quite plausible that Winston trusts O'Brien on the basis of a look in the Party member's eyes ?
First, 1984 is a portrayal of an extreme example of a post-truth or post-fact society. The members of the society such as Julia and Winston who have some understanding of the Party's modus operandi come to realize that people and information are inherently untrustworthy and that even history is constantly rewritten. The language of "doublespeak," in fact, attempts to make it impossible to think or speak the truth even if one desires to do so.
With no external objective measures of truth available to her, Julia trusts the only things she has left, her emotions and instincts. When language fails to be an instrument of truth, immediate emotional intuitions and connections are all that remain. Also, because of the ubiquitous surveillance in the society, anyone attempting to communicate something that might be subversive must do so through very subtle methods such as quick glances or subtle gestures rather than more overt forms of communication.
The reason for this is that Julia is much more of an impulsive and emotional person than Winston, in my opinion.
Over the course of the book, we see that Winston is more interested in political things than Julia is. He wants to overthrow the Party because he dislikes its political policies. He wants to have more freedom and fewer lies. By contrast, Julia wishes the Party were gone so that she could have more fun on a personal level. She is not really interested in abstract ideas of rights.
Because Julia has this sort of a personality, she is more impulsive. Therefore, we are told, she often judges people like that -- just on some sort of a hunch. Here is a line that shows this:
She was used to judging people by their faces, and it seemed natural to her that Winston should believe O'Brien to be trustworthy on the strength of a single flash of the eyes.