“When someone shows you who they are believe them; the first time.”

As quoted by Oprah Winfrey.
Oprah Winfrey's paraphrase:
When people show you who they are, believe them.
Source: [When people show you who they are, believe them, http://www.oprah.com/oprahs-lifeclass/when-people-show-you-who-they-are-believe-them-video, 2016-01-19, Oprah's Lifeclass, Oprah, Winfrey, Oprah Winfrey Network, 2011-10-26, 1, 13, 90, en-us, Oprah Winfrey]
Source: [Book club finale, http://www.oprah.com/oprahshow/One-of-Dr-Maya-Angelous-Most-Important-Lessons_1, 2016-01-19, The Oprah Winfrey Show, Oprah, Winfrey, 1997-06-18, 45, en-us, Oprah Winfrey]

Source: Blue-Eyed Devil

Source: The 7 Habits of Highly Effective People: Powerful Lessons in Personal Change

Source: The Foundation series (1951–1993), Foundation’s Edge (1982), Chapter 11 “Sayshell” section 3, p. 205
Source: Foundation's Edge
Context: Pelorat sighed. “I will never understand people.”
“There’s nothing to it. All you have to do is take a close look at yourself and you will understand everyone else. We’re in no way different ourselves... You show me someone who can’t understand people and I’ll show you someone who has built up a false image of himself.”

Chuck Klosterman IV: A Decade of Curious People and Dangerous Ideas (2006), Recognizing Your Archenemy

Source: The Wine of Solitude

Source: Why Men Love Bitches: From Doormat to Dreamgirl—A Woman's Guide to Holding Her Own in a Relationship