What does the line in Macbeth mean:"There is no art/ to find the man's construction in the face"

King Duncan makes this observation in act 1 sc. 4 when
Malcolm reports that the Thane of Cawdor who betrayed the king has been put to death.
King Duncan means to say that what is hidden in the mind of a man can never be
understood by looking at his face. That is to say, the face is never a true index of the
mind. The king believed the Thane of Cawdor to be very trustworthy, but he secretly
helped the Norwegian king against Duncan.

However this
piece of comment also becomes an example of irony. As Duncan says this, Macbeth enters
with Banquo and others. Having gone through the previous scene--act 1 sc. 3--we know
that Macbeth, Duncan's most admired and trusted general, is tempted, by his own ambition
and the prophecies of the witches, to kill the good old king and usurp the throne.
Duncan has declared Macbeth as the new Thane of Cawdor, but it is an irony that this
time it is going to be a worse kind of betrayal. Truly the 'mind's construction' can
never be known from the face.