Words used in the title can often give you clues about a book's intended audience or purpose. Scholarly book's usually have titles that sound more formal. For example, books with titles like Complexity and Evolution: Toward a New Synthesis for Economics have a much greater likelihood of being scholarly as compared to books with titles like Business Cat: Money, Power, Treats.
Timeliness and currency are important when selecting a book for your research. Whether or not a book is considered current will vary greatly depending on the discipline and the particular topic. Additionally, since it can take years to write and publish a book, if you're looking for the most up-to-date information available, you may need to consult other types of sources that have a faster publication cycle.
Authors of scholarly books cite their sources using formal citation styles like APA, MLA, Chicago, or others. These references may appear as parenthetical citations, footnotes, endnotes, or bibliographies. When evaluating the quality of a scholarly book, it's often useful to examine these citations to get a better sense of the relationship between the book and other scholarly work on the topic.
The text of scholarly books will usually have a more serious tone and use formal or technical language that may not be easily understood by a general audience.
Scholarly books are almost always written by professors or researchers affiliated with universities or research institutes. Some scholarly books, often called edited books or edited volumes, are co-authored by a group of people who are each responsible for a particular chapter. The editors of edited volumes are also professors or researchers.
Most scholarly books list their authors' and/or editors' credentials (i.e., affiliations with universities or research institutes, previous publications, and sometimes academic degrees). This information can be useful in determining whether someone is considered an authority on the topic they're writing about.
There are several different categories of publishers, each specializing in particular types of book. A quick Google search can tell you which of the following categories a publisher belongs to:
Scholarly books, e.g. Harvard University Press
Professional, technical, reference, textbooks, e.g. Wiley, Scholastic
Scholarly (often reporting research produced by members of the center or institute), e.g. Pew Research Center; Brookings Institution Press
Popular/mass-marketed, e.g. Random House, Penguin
Scholarly, Textbooks, e.g. Muse Open, National Academies Press
Reviews of scholarly books are often published in scholarly journals. If the book you're evaluating has been reviewed, it can be helpful to consult the review to get a better sense of how the book is perceived by other scholars doing research in the same subject area. You can find book reviews in many library databases: