Use this checklist to determine if a source is appropriate:

Authority: Who wrote it? What credentials does s/he have? [PhD, affiliation with university]

Source: Where was it published? How was it selected for publication? [look at submission guidelines for evidence of peer review or other editorial processes]

Currency: When was it published? Has it appeared in other forms with/out revision?

Research: Are other sources cited in a bibliography/reference list? Are foot/endnotes used?

Bias: Is there an objective viewpoint? Is the author making assertions without documentation, or with questionable documentation? [e.g., her/his own work, crackpots]