How do I know if a website (or other source) is good?

Use the CRAAP test!

The CRAAP test is an acronym that you can use to remember important criteria in evaluating resources. You can use the method for ALL resources, but this lesson will focus on Web resources. For a brief introduction to the method, check out this video:
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Source: http://genconnection.com/English/ap/EvaluatingSources.htm
Still unsure about CRAAP test? Check out this comic (source THETWELVES at Stripgenerator)