The TechStars program does this really well. They promise you brutal honesty. They tell you to brace for it. You are guaranteed to be told when your idea sucks. You can see this in practice each week as the teams present their pitches. Because honesty is the expectation, the quietest guy in the room will raise his hand and say, "For the first ten minutes, I had no freaking clue what you were talking about." Ouch! But the permission was there. The guy wasn't a jerk, he was embracing the culture. And you know what? It needed to be said.
In the early days, Rackspace had a saying: "Bad news first, full disclosure, no surprises." More than a slogan, it was permission to be honest. People would say, "It sucks to bring this up, but as we say, bad news first and full disclosure..." Again, amazingly effective.
People want to be honest. And in the end, they want you to be honest, too. There just needs to be permission.