The publisher can be:
Most scholarly - a university, professional, or academic press; for example: Oxford University Press, American Psychological Association, Elsevier
Mid-level - reputable book publisher, magazine, or newspaper
Least scholarly - self-published/vanity press
Learn about publishers by visiting their websites and looking in the "About" section.
For a source you're considering: Where does the publisher fall on a scale of 0 to 10?
To find out who the publisher is, look either on the page near the front of the book that gives the title, or look on the page that has the copyright information.
You can learn basic information about a magazine, journal or newspaper by visiting that periodical's website. Look for an "About Us" section.
You can also learn more from Ulrich's Global Serials Directory (link is below). It usually provides a description, intended audience, and sometimes reviews.