If you have a scholarly information source like a monograph or a peer reviewed journal article, you're in luck. When these kinds of information sources are published, they have your needs in mind.

So you know: 

• It was either peer reviewed or editorially reviewed (and you can find out more by going to the publisher's or journal's web site.)
• It was produced by a subject expert and for an audience of subject experts.
• It was produced for the purpose of informing and educating. It is supposed to be objective and unbiased. (It's not perfect, but it's still better than if there were no such expectation!)

Plus, they make it very easy to find out:

• Who wrote this?
• When was this written? 
• Who provided funding for this?
• Who published this?
• What potential conflicts of interest are there?
• What kinds of sources did they get their information from?

Non-scholarly but formally published information sources

Here are some types of information sources that are formally published but non-scholarly:

• Newspapers
• Documentaries
• Magazines
• Books from commercial publishers.

For these kinds of information sources, you will easily be able to find out:

• When it was written?
• Who wrote it?
• Who published it?
• Maybe what sources they got their information from.

But non-scholarly information sources are not held to the same standards as scholarly ones. There is a whole range, from highly credible to thoroughly disreputable. 

• They may or may not be objective and unbiased.
• They may or may not attempt to mislead or manipulate you.
• They may or may not use good sources and check their facts.

You will have to make a judgment based on two things:

• The reputation of the organization that published the information source (book publisher, magazine, etc.)
• The actual contents of the information source.

Information sources that were never formally published

It used to be that without the resources of a large organization like a journal or book publisher, it was hard for an author to get their information source out to an audience. So almost all research was done using books and articles (and of course, primary sources, which were sometimes very hard to get to.)

Now we have the Internet, and if you have a connection, you can make a web page and put whatever you want up there. If you go to college, your college may give you an opportunity to put your papers into an online repository. Organizations and companies of all kinds put many of their documents online - everything from policies and procedures, to data and reports. Ephemera like brochures and pamphlets that used to be throw-away pieces of paper now linger on the web for years after they've gone out of date. Famous authors have blogs, and so do politicians, subject experts, and schoolchildren, and business people trying to make a name for themselves. 

All of these things are available to you as information sources. Some of them may be good for research, and many others are not. 

• They don't have a standard way to tell you who created them (and they may not tell you at all.)
• They don't have a standard way to tell you when they were created (and they may not tell you at all.)
• They don't have a standard way to disclose any sources of financial support or biases (and they may not tell you at all.) 
• They don't have a standard way to tell you what information sources they used, if any (and they may not tell you at all.)
• They may actually be plagiarizing text or images or even copying whole other sources and not giving you any way to find out. 
• Depending on who produced the content, there may not be oversight from a larger organization with a reputation to worry about, which might have helped to ensure quality and ethical integrity of the content.

Plus the useful informational content is mixed in with content that was made to sell you things, manipulate you into changing your mind or changing your actions, and straight up deceive you. 

Long story short, you have to be really careful with information sources that were never formally published. Usually that means "websites," but the same rules apply to:

• self-published or vanity press books
• public access TV shows
• pamphlets, newsletters, etc. produced by an organization to promote itself or its cause.