Why does Julia find it quite plausible that Winston trust O'Brien on the basis of a look in the Party member's eyes ?

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.