While not always the case for every source, credible sources generally aim to inform, using language that is neutral, unbaised, and unemotional.
Any source that shows evidence for being accurate and trustworthy is a credible source. A credible source could be a book, website, newspaper article, magazine article, journal article, etc. if the source shows evidence that it is trustworthy.
What evidence is there for credibility?
Is the source scholarly or peer-reviewed.
Any source that is scholarly or peer-reviewed is generally considered credible.
Look for reputable names (and consider their reputation).
Look for names of authors, partners, sponsors, participants, or board members. Or, check the name of publisher, the parent institution, etc. Do you recognize any as being reputable?
Examples: the American Psychological Association or an author whose credentials include working at university.
Is the source's author likely to be concerned about their reputation for being reliable and accurate? And, are there likely to be negative repercussions if their content is low-quality?
Look at the author's credentials. Is the author in a "position to know" about the subject matter?
Think about the audience (and consider their motives).
Was the source created to inform, persuade, create chaos, entertain, etc.?
Look for research evidence.
Does the author of the source back up their claims with relevant evidence? Evidence includes references (also known as citations) to other, credible sources.
A few other things to consider:
Examine the writing style and grammar - it should be professional.
Try fact-checking the information (i.e. lateral reading) to see if you can find the same information corroborated in other sources.