A good approach to evaluating sources is to ask yourself a series of questions that address: Currency, Relevance, Authority, Accuracy, and Purpose (CRAAP). This process not only helps you determine credibility but also relevance. (printer-friendly pdf)
Currency: To determine if the date of publication of the information is suitable for your speech.
What is the copyright, publication, or posting date?
Does the date matter? Is the information outdated?
Relevance: To determine how applicable the information is for the purpose of your speech.
For what audience or level is the information written (general public, experts/scholars, etc.)?
Explain why you would or would not quote/reference the information from this source in your speech.
Authority: To determine if the source author, creator, or publisher of the information is the most knowledgeable.
Who is the author, creator, or publisher of the source or what organization is responsible for the source?
How do you know if the author is an expert on the topic (e.g examine the author’s credentials and/or organizational affiliation)?
Accuracy: To determine the reliability, truthfulness and correctness of the content.
What indications do you see that the information is or is not well researched or provides sufficient evidence?
What kind of language, imagery and/or tone is used (e.g. emotional, objective, professional, etc.)?
Purpose: To determine the reason why the information exists.
Why was this source written (e.g.to inform, teach, entertain, persuade)?
How might the author's affiliation affect the point of view, slant, or potential bias of the source?