I recommend that everyone read this article by Robert L. Park. The Seven Warning Signs of Bogus Science. It's old but still applies.
We are constantly bombarded with claims every day. Our friends, our television and our computers are filled with "miracle" cures, get rich quick schemes and dieting hooey. By knowing these signs, you'll be better equipped to quickly evaluate extraordinary claims before you lose your money or your health.
This article is really just a good place to start. I highly recommend that everyone read The Demon Haunted World: Science as a Candle in the Dark, by Carl Sagan. Here is the synopsis from Amazon:
Carl Sagan muses on the current state of scientific thought, which offers him marvelous opportunities to entertain us with his own childhood experiences, the newspaper morgues, UFO stories, and the assorted flotsam and jetsam of pseudoscience. Along the way he debunks alien abduction, faith-healing, and channeling; refutes the arguments that science destroys spirituality, and provides a "baloney detection kit" for thinking through political, social, religious, and other issues.
Thanks to the Bad Astronomer for the tip on Park's article.