Does it have footnotes or endnotes that are mostly references to other works? That's a pretty good sign of an academic book. Popular books will often have a lot fewer footnotes, and the ones they have will focus more on defining terms or giving extra information that an academic audience would be more expected to just know.

Check the publisher--some presses are academic only or popular only, and very often the ones that do both have separate imprints. If the book is part of a series, you can Google the series and see if it's academic or popular. If you're in college/university, check the library catalog for book reviews (search the title+author, and then narrow the search to articles--reviews will be journal articles with the title of the book, with the author's name usually either as part of the article title or listed on the author line, depending on how the particular journal does it). Good book reviews will say something about the intended audience. (If you don't have access to a university library system, Google can be your friend, but most reviews in academic journals are behind paywalls.)

Also pay attention to the content and style. If you're not familiar with academic writing, a lot of times it will be more difficult to read than writing for a popular audience. Popular writing will often have a more "narrative" feel, designed to keep your interest. Academic writing will often be more technical-sounding, and it's designed to convey information or an argument more than to be engaging (obviously, what this means depends on the field, and it can take practice to distinguish them. And good academic writers can be both engaging and technical). Academic writing will often use a lot more technical terms, where popular writing will either use more colloquial language or define the technical terms when they're needed.

If you're in a field that uses non-English sources (like primary sources in Latin or Arabic, or secondary scholarship in German), a lot of times academic work won't bother to translate, especially single words and short phrases, or will include both the original language and a translation. Popular books will often just give you the translation without the original language.

That's the main stuff. If you're in school, ask instructors for more field-specific things to look for, or go spend a few minutes with a reference librarian to get see how your school's library catalog works and get more specific advice for what you're doing.