There is no simple formula for evaluating sources; evaluation always depends on the facts of your own rhetorical situation.  

Here is a basic framework you can use to evaluate your rhetorical situation and analyze how well your sources support it:

1. Is the source useful to you?

• Does it provide the kind of information you need?
• Does it meet your assignment requirements?
• Does it make you think? Did it spark further questions or suggest additional lines of inquiry?
• Does it help you contextualize or understand other sources?

2. Is this the type of source your audience expects you to use?

• Is it at the right level -- not too difficult nor too easy for your audience? 
• Will it give you more credibility with your audience if you use it?

3. Who created the source?

• Is the author identified AND if they are, are they someone you find credible?
• If the author is not identified, is there a group or institution responsible for the source?  Do you find that group credible?
• Have you done whatever additional research you need to do to decide if the author is credible or useful? 

4. What is the author's (or institution's or agency's) purpose in creating this source?

• Are they trying to persuade you to do or think something specific?
• Are they selling something?
• Does their purpose or agenda affect the quality of their evidence? Did it affect how they presented it?

5. (If the source is a scholarly one) is it a good example of research in the discipline?

• Do they use citations, footnotes, etc. to connect their work to the rest of the field?
• Is the source referenced by other scholars you've found?
• Is it current enough to meet your needs?
• Was it published in a journal or other source important in the field?

(This framework is adapted from one created by OSU librarian Anne-Marie Deitering in The Academic Writer, by Lisa Ede)