How can you tell if something is a scholarly source? Look for these points:
Written by someone educated in the field of knowledge they are addressing.
For example, a professor, graduate student, an independent researcher with degrees in the same field they are writing about, or someone with equivalent qualifications within their domain. Generally not: Journalists, amateur enthusiasts, etc.
Include citations for the facts, figures and theories they use.
Academic scholars are required by standards and ethics to cite the sources of information that they use. Check for footnotes, a bibliography or works cited page. No citations = probably not a scholarly source.
Ideally, has been vetted by other scholars within their field in the process called "Peer Review."
Most academic journals and academic presses employ a process called Peer Review for their publications. In this process:
An author submits their paper to an academic editor
The editor hides the name and other identifying features of who the author is, and sends it to two or more other scholars from the same field for review.
The other scholars (who also remain anonymous to the author) review the paper for errors, omissions or other faults. They submit their notes to the editor with a recommendation to "Publish as-is," "Publish with Revisions," or "Not recommended for publication."
The editor uses the reviewers' recommendations to make a decision, and communicates it to the author.
If accepted, the author usually has to make at least some revisions before publication. This ensures that major errors, big gaps or other problems are more likely to be corrected vs. non-scholarly publication.
How can you tell if something has been vetted? The short-cut is to look at who is the publisher. Generally, look for:
Academic journal articles, like those in the databases linked in this site. HOWEVER: Make sure that you are choosing a proper JOURNAL ARTICLE, and not an opinion piece, an editorial, or a short news piece, etc. Proper journal articles are usually at least 10 pages long, have citations, and make an argument backed with facts rather than just opinions.
Academic press books. These usually are named "University of X Press" or similar.
Sometimes, non-academic press books also qualify; but take a careful look at who the author is for these books. Most popular press books, magazines and newspapers are not scholarly. Again, check for citations, use of facts, etc.