Malcolm describes Cawdor's last moments before execution, what is Duncan's reply?

The answer you are looking for can be found at the start
of Act I, Scene 4.

In that scene, Malcolm tells Duncan that
the Thane of Cawdor died well.  He begged for the king's forgiveness and apologized for
what he had done (he became a traitor and helped the enemy in the battle that had just
finished).

When Duncan hears this, he is not really all
that impressed.  Basically, he just says there's just now way to know what a person is
really like by looking at them.  He says he trusted Cawdor and that Cawdor's actions
proved he was wrong to do that.

Here's the
quote:

There's
no art
To find the mind's construction in the face:
He was a
gentleman on whom I built 290
An absolute
trust.

Ironically, enough,
Duncan then turns around and puts his trust in Macbeth, making him Thane of Cawdor. 
That doesn't turn out so well for Duncan
either...