Books in Primo are assigned Library of Congress Subject Headings. In many ways, subject headings are a form of tagging, in that they represent the content of the material and provide ways for you to efficiently locate more materials that are conceptually related.
Here are several examples:
Here are some clues to look for in the catalog record when you are evaluating whether a book is scholarly or popular:
When you have the book in hand, and still aren't sure if it is scholarly, you might want to do a little more digging, perhaps with a couple of quick Google searches: