In general, reliability means consistency, quality and integrity.
In the case of a scholarly information source, like a monograph or scholarly journal article, 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 website).
- It was produced by a subject expert for an audience of subject experts.
- It was produced for the purpose of informing and educating. It is supposed to be objective and unbiased (though this is sometimes not the case).
Plus, with scholarly information sources, it is easy to identify:
- Who wrote it?
- When was it written?
- Who provided funding?
- Who published it?
- What potential conflicts of interest are there?
- What kinds of sources were used in the research?
You can learn a lot about an information source by looking at the the organization(s) responsible for producing it. Some questions to ask when encountering non-scholarly information sources:
- Is their reputation for putting out good information their first priority, or do they have other priorities? In particular, be wary of any organization that is trying to sell something, raise money for something, win an election, win a court battle, win a war, win a battle of public opinion, save souls, or save the world. Not that there is anything wrong with those things necessarily, but it is very hard for anyone to put objective scholarship first when they have those priorities.
- Do they value honesty and integrity?
- Do they value research and scholarship?