In order to determine if the eggs are fertile, crack one open gently (don't break the yolk) Look on the yolk of the egg, if there is a small white dot in the very center of the egg that means that your eggs are fertile. If you can't see the small white dot at first try to keep turning the egg yolk over gently with a spoon to see the dot.
Ducks also need water in order to mate. Make sure you have deep enough water for them to mate in. If you are unsure if you have a male and female. The female duck will usually be missing some feathers on her neck. The male will have one curved tail feather.