Malcolm is suspicious of Macduff when he arrives in England because he is not sure if he has come as a spy for Macbeth. When Macduff asks Malcolm to join him in an effort to rescue the ailing Scotland:
Macduff: Hold fast the mortal sword, and like good men
Bestride our down-fall'n birthdom: ...
... new sorrows
Strike heaven on the face, that it resounds
As if it felt with Scotland and yell'd out... (Act IV, Scene III)
Malcolm replies that Macduff might be after a deserved reward from Macbeth as a result of the "innocent lamb" that is Malcolm sacrificed to Macbeth's ambition:
Malcolm: This tyrant, whose sole name blisters our tongues,
Was once thought honest: you have loved him well.
He hath not touch'd you yet. I am young;
You may deserve of him through me, and wisdom
To offer up a weak poor innocent lamb
To appease an angry god. (Act IV, Scene III)
And the other reason he is not sure of Macduff is because Macduff has left his wife and children back in Scotland. A fact that gives Malcolm reason to believe that Macduff is confident of his security in that situation.
Malcolm: Why in that rawness left you wife and child—
Those precious motives, those strong knots of
Without leave-taking? I pray you,
Let not my jealousies be your dishonours,
But mine own safeties:... (Act IV, Scene III)
Malcolm questions Macduff's feelings for Macbeth and why he has left his wife and children in Scotland.
When Macduff travels to England to ask for Malcolm's assistance in displacing Macbeth, Malcolm makes himself out to be a bad choice for king. He tells Macduff that he has insatiable lust and greed. He says he has no interest in the kingly duties, and ends by telling Macduff that he would certainly destroy peace in Scotland:
Malcolm: But I have none: the king-becoming graces,
As justice, verity, temperance, stableness,
... Nay, had I power, I should
Pour the sweet milk of concord into hell,
Uproar the universal peace, confound
All unity on earth. (IV.3)
Macduff is immediately distraught by this news. Having come to Malcolm in hopes of restoring a rightful ruler to the throne and ensuring the safety of the Scottish people, Macduff cries out that his country is doomed:
Macduff: Fit to govern!
...O nation miserable,
With an untitled tyrant bloody-scepter'd,
When shalt thou see thy wholesome days again,
These evils thou repeat'st upon thyself
Have banish'd me from Scotland. O my breast,
Thy hope ends here!
The passion which Macduff shows in the love of his country, and the fact that Macduff gives up on Malcolm once Malcolm insists he would destroy peace is the proof needed. Malcolm understands that Macduff has come to him out of patriotism, and not out of glory for himself.
Malcolm: Macduff, this noble passion,
Child of integrity, hath from my soul
Wiped the black scruples, reconciled my thoughts
To thy good truth and honour. [...]
...and modest wisdom plucks me
From over-credulous haste: [...]
I put myself to thy direction,...
Malcolm is afraid, but he does not know exactly who is friend and who is foe, so he tests Macduff by pretending to be a little crazy and extremely unworthy of the throne. He pretends to be irresponsible and greedy.
Macduff does not waver, however, in his belief that Malcolm is the rightful heir to the throne and certainly a better choice than bloodthirsty Macbeth.