WE HAVE FERTILE DUCK EGGS!
After trial and error, look for the bullseye when you crack a fresh egg open. If it’s a white dot with no bullseye, the eggs are most likely not fertile.
Here is what a fertile duck egg looks like:
This is the second egg we cracked open and saw a bullseye. The rest are going in the incubator!!!
This is what one of the eggs looked like when it wasn’t fertile yet:
I found this chart below to be super helpful: