How can you tell if the nonfiction you are reading contains opinion?

headings will indicate any use of opinion
only fiction makes use of opinion in writing
there will be signal words and phrases
it is best to write to the author to find out

to leave the reader thinking and wandering on their own. without a resolution we are clueless to what happens next, and why it does. it may even cause suspension with the reader.

explanation:

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.