I had one floater -- an indication my other research told me was rotten. And two that stood up straight, meaning they were old and would not be great for poaching (which is what I wanted them for, poaching them along side minted lamb meatballs in a Moroccan-style tomato sauce), but woudl be fine for baking.

Does it quack?

Oh yes. Smell is the way to go! Yes, the properties of the egg may change, but it should be considered inedible once it begins to smell of well, rotten eggs (sulfur).

Just an educated guess until a duck egg expert comes along, but I'd think it would be the same as with chicken eggs. Eggs rarely go bad, they just shrink up inside the shell and the white gets watery and doesn't sit up as high as a fresh egg. You will know if it is bad by the smell. So if it isn't bad it is just old but edible. It may not be the best choice if you are making something where you rely on the egg white as a rising agent (like souffle.)