How to find credibility of a quote

The challenge of the day is finding credibility of a quote attributed to Dr. Martin Luther King:

"I mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy."

In Google Power Search, Prof. Daniel M. Russell, Google's resident search anthropologist, teaches how to, a) Verify the credibility of information you find on the web, and b) To check your findings, just do one more search. He says: "Credibility--can you trust the information you find online? How can you find out whether information is accurate and true?"
Interestingly, on the Internet every creative sentence sounds interesting. But, most lack the required credibility--who said, when, how and where it can be verified.
Here are few researched articles, identified by Dan, on the ways to find if a statement is real or fake. Each of these relate to finding if a quote attributed to Dr. Martin Luther King was authentic:

Google Date Range Filter Simplifies Search Results, By Robert Strohmeyer, PCWorld

Anatomy of a Fake Quotation, by Megan McArdle

Out of Osama's Death, a Fake Quotation Is Born, By Megan McArdle

How to Spot a Fake, by Anna Berkes
On the same shelf: Where else can you verify a quote, check the following:

www.quoteworld.org

www.thinexist.com