Duck eggs fertile?

You can crack some open if you know exactly what to look for- but test incubating some is the surer way to go.

 

There are two ways to do this. The first way is to incubate them for 7-8 days and check them for blood vanes. The second way is checking the yolk to see if the germ has divided. You do this by cracking the egg open and inspecting the surface of both sides of the yolk for a light spot. All eggs have one, unfertile eggs have a very small germ You are looking for a spot that is about 1/8th of an inch wide this indicates that the egg is fertile. You will want a magnifying glass and a bright light handy because sometimes it is hard to see.

 

You can check for fertility without incubating, but personally I wouldn't sell eggs for hatching unless I've run a test batch myself. There are too many variables and I'd be worried I was selling bad eggs. You just can't know until you've hatched some.

Alternately, if you really don't want to incubate them yourself, you can offer a dozen at a discounted rate, or free, to someone willing to run a test batch for you and report back on their results. Less reliable (because you can't control conditions), but works okay anyway.

Here is a thread, with pictures, showing how to tell fertile from infertile by cracking them open:

https://www.backyardchickens.com/forum/viewtopic.php?id=16008

Good luck!