Hamlet doesn't ever expressly admit where Polonius's body is, but Act 4, scene 3 gives some veiled hints.  Hamlet hints, in a not so subtle way, that Polonius is dead and rotting.  Claudius asks Hamlet where Polonius is, and Hamlet responds "At supper."  Of course that line makes it sound like Polonius is alive and well, which is what Claudius thinks.  Then Hamlet tells an utterly disgusting story about Polonius at dinner.  

Not where he eats, but where he is eaten. A certain convocation of politic worms are e'en at him. Your worm is your only emperor for diet. We fat all creatures else to fat us, and we fat ourselves for maggots. Your fat king and your lean beggar is but variable service—two dishes, but to one table. That’s the end.

In other words, Polonius is at dinner, because he is dinner.  The worms are eating him the same way that worms eat beggars.  

Claudius still isn't sure what Hamlet is talking about, so Claudius asks Hamlet again where Polonius is.  Hamlet lays it on quite thick now and flat out tells Claudius that Polonius is either in Heaven or Hell.  

In heaven. Send hither to see. If your messenger find him not there, seek him i' th' other place yourself. But if indeed you find him not within this month, you shall nose him as you go up the stairs into the lobby.

The last part of that line is referring to the stench of Polonius's rotting carcass.  Essentially, Hamlet tells Polonius to follow his nose in order to find the dead body.