How can you tell whether a work is peer-reviewed? First, there are certain clues in the article itself:
1. Author credentials: They are an expert in the area they are studying and has a degree (PhD, for example) and a job (in academia or a research institute, think-tank, hospital, etc.) that relies on that expertise. When in doubt, Google the author!
2. Publisher: Usually the journal will be published by a scholarly society, university press, or major scholarly publisher like Elsevier, Springer, Taylor and Francis, Wiley, etc. When in doubt, Google the journal title!
3. References: The authors of peer-reviewed articles will show you where they got their information from, either in the form of footnotes at the bottom of each page or a bibliography/endnotes at the end of the article.
4. Look: Peer-reviewed journals have a plain appearance. If they contain images, they tend to be figures, charts, and other images critical to understanding the research.
5. Language: The authors of peer-reviewed articles are writing for experts in their field, so the language is dense and discipline-specific.
If you found the journal article on the web, try Googling the journal title. Usually somewhere in the description or the instructions for authors, it will mention whether there is a peer-review process.
Note that an article can be from a peer-reviewed journal and not actually be peer reviewed. Editorials, news items, and book reviews do not necessarily go through the same review process. A peer-reviewed article should be longer than just a couple of pages and include a bibliography.