how do i know when?

If you don't see them, crack one of their eggs and see if it has the fertile ring, or incubate some

 

If you have the opportunity to watch them in a pool, you will see the drake and hen bobbing their heads up and down. That starts the courtship. Then the hen will stretch her neck horizontally along the water to invite the drake. The drake will then climb on her, peck her on the back of the neck and head and sometimes submerge her in the water. When he's done, he hops off and the hen comes up for air.

First time I saw this with my ducks, I heard a gagging sound which made me look. They were new at it so the hen wasn't ready to be dunked!

BTW, it is easier to mate in the water than on land, but it can be done on land as well, just not very graceful.