Polonius is told by Claudius. In 3.1.31-40, Claudius seeks to prove his allegations to her father. He first dismisses Gertrude so that they might have a word alone. He offers to let Polonius spy on the pair (lawful episals: they are not doing anything wrong, the are legally spying on children who are under their dominion):

"Sweet Gertrude, leave us,
For we have closely sent Hamlet hither,
That he, as 'were by accident, may here
Affront Ophelia.
Her father and myself (lawful episals)
(Will) so bestow oursleve that, seeing unseen,
We may of their encounter frankly judge
And gather by him, as he is behaved,
If 't be th' affliction of his love or no
That thus he suffers for.