Content is published in a wide variety of formats and for diverse audiences.
You should always make your own assessment of the reliability, significance, and purpose of the literature you engage to support your own work. But here are some general guidelines.
In the form of books or articles:
If you're after scholarly articles, then limit your searches in databases to material that is peer-reviewed. Here's a quick summary of what 'peer-reviewed' means: articles in scholarly journals have been evaluated (reviewed) by a committee of scholars (peers) before publication.