What is the role of rhetoric in influencing people's attitudes and
beliefs? How can readers distinguish between prejudicial and
non-prejudicial use of rhetorical devices?

Just a free comment:

For a start you could search here with:  define:rhetoric   (no spacing)

You will find many definitions, plus interesting links at the top of the page.

Rhetoric is generally considered to relate to speaking rather than writing.

As to "prejudicial" vs "non-prejudicial":  because the intention of
good rhetoric is to improve the effectiveness of the speaker's
argument/presentation, I feel that it is seldom if never
non-prejudicial.  Even just presenting the facts objectively  - from
the speaker's point of view -  in the best way possible, is
prejudicial  (but someone may disagree with me).

Of course, there are "tricker" rhetorical devices, such as rephrasing
a question and then answering that instead of what the questioner
actually asked  (much favored by politicians).

Hope this helps a bit.