The fastest way to make a team honest is for the leader to admit a mistake first.

Groups take their reading on safety from whoever is at the front. Until somebody with standing admits something, everybody else is working out what it would cost them, and the sensible answer is to stay quiet. One honest admission from the leader resets that calculation for the whole room in about ten seconds.

If you lead anything, go first and go specific: the thing you got wrong and what you are doing about it. Then leave a pause instead of filling it. What happens next tells you a great deal about what people have been holding back.