The right question can make someone tell themselves the answer.

A well-aimed question can do what no argument can, because a conclusion someone reaches themselves does not have to be defended against. You are not delivering an answer; you are opening a line of thought that they follow to a place they had not looked.

When you can see something someone else cannot, resist telling them. Ask instead about the part they have not examined. What would have to be true for this to work, or what would you tell a friend in this position. Then be quiet. The answer they give themselves will hold far better than yours would.