Judge a man by his questions rather than his answers.

Voltaire (1694-1778) French writer [pseud. of Francois-Marie Arouet]

(Misttributed)

Actually Pierre-Marc-Gaston, duc de Lévis (1764-1830): "Il est encore plus facile de juger de l'esprit d'un homme par ses questions que par ses réponses." [It is easier to judge the mind of a man by his questions rather than his answers] in Maximes et réflexions sur différents sujets de morale et de politique, Maxim 17 (1808)