Trust, but verify

Trust, but verify is a form of advice given which recommends that while a source of information might be considered reliable, one should perform additional research to verify that such information is accurate, or trustworthy.

The term was a signature phrase adopted and made famous by U.S. president Ronald Reagan. Reagan frequently used it when discussing U.S. relations with the Soviet Union. The phrase was learned by Reagan from Suzanne Massie, a writer on Russia. She told Reagan, "The Russians like to talk in proverbs. It would be nice of you to know a few. You are an actor, you can learn them very quickly".

After Reagan used the phrase at the signing of the INF Treaty, his counterpart Mikhail Gorbachev responded: "You repeat that at every meeting," to which Reagan answered "I like it”.

From http://en.wikipedia.org/wiki/Trust,_but_verify