It is in Act IV, Scene 3 that Malcolm, son of Duncan, tests the nobleman Macduff.
Realizing that his father, King Duncan, died because he trusted Macbeth, Malcolm has become skeptical and wary of others. For this reason, Malcolm seeks to ascertain that Macduff has not been sent by Macbeth; so, in order to test Macduff's loyalty to Scotland, Malcolm pretends to have many vices. Scene 3 opens with Malcolm and Macduff talking in private:
Macduff immediately declares his love for Scotland and the dire situation of the country in which
New widows howl, new orphans cry, new sorrows
Strike heaven on the face...(4.3.5-6)
But, Malcolm questions Macduff's sincerity by responding that Macduff was once one of Macbeth's favorites. He wonders aloud if Macduff wishes to betray him to Macbeth. Then, he asks Macduff why he has left his wife and child to come to England, seeking him. Macduff responds that Scotland is bleeding from the tyranny of Macbeth, and it needs Malcolm to take the throne as the son of King Duncan. Malcolm explains that he is merely trying to protect himself. Further, he describes his faults as worse than Macbeth's:
It is myself I mean, in whom I know
All the particulars of vice so grafted
That, when they shall be opened, black Macbeth
Will seem as pure as snow, and the poor state
Esteem him as a lamb, being compared
With my confineless harms. (4.3.51-56)
But, Macduff remains steadfast in his purpose and insists that Malcolm is the rightful heir and must return to Scotland. Convinced of Macduff's loyalty to his country, Malcolm reveals that he has only been testing him, and he promises to return.