How do I know when my goose want to sit on eggs

I though I had a male and female goose and when one died I though the female died. But then I started finding unusually large eggs in the duck house, albeit in a new nest. Today I peeked in to watch and she was arranging her nest. What behavior should I look for to make me assume she wants to sit? And where to get I get a male that can fertilize the eggs she wants to sit on? She's an Embden.

Sandy in Florida