How does Malcolm test Macduff's integrity and loyalty?

Macduff has fled to England to escape Macbeth's wrath, and he seeks out Malcolm there. When the two meet, Macduff is hoping that Malcolm, the rightful heir to the throne as Duncan's son, will return to overthrow Macbeth and rescue Scotland from the tyrannical rule of Macbeth. But Malcolm tells him that he is actually a worse man than Macbeth in many ways. He is a drunkard, a womanizer, and corrupt. He says that he has a "stanches avarice" that would lead him to steal from the nobles were he made King. He concludes that he has none of what he calls the "king-becoming graces," which include such traits as "justice, verity, temperance" and "stableness," to name a few. When Macduff hears this, he loses hope for Scotland. When he expresses his despair, Malcolm reveals that he is actually a good, honest, chaste man--he was simply saying otherwise to determine where Macduff's loyalties lay. Macduff's response convinces Malcolm that he is to be trusted, and the two determine to join forces to overthrow Macbeth. Macduff's resolve is cemented later in the scene, when Ross arrives to tell him that his wife and children have been murdered at Macbeth's order.