Post pictures with something to compare size with. My favorite is a soda can, it is a universal size that most people can compare to. Maybe by getting pictures, someone can tell whether they are domestics or not. You may even have rouens or something.
I would even call the lady you got them from and flat out ask her. "Hi, I was wondering if there is any possible way that these could be domestic ducklings? Or even come from a domestic bred mallard?" You could feed her some story. "Fish and Game will not let me keep them unless they are domestics, and I would hate to turn a non-wild animal loose into the wild to be eaten by predators or starvation"
You have the ducklings now, the only one that is going to lose because of the "I rescued them" story is going to be the ducklings.
I don't want to advice you to do anything illegal, but if it were I with the ducklings. I would simply raise them as any other duck, and if they were actually fully flighted mallards let them do what they will once they become of age. As long as you don't cover their pen or pinion them, they will fly away as they please if they really are wild mallards. Then if they turn out to be a domestic duck, such as a rouen, they will be unable to fly away.
-Kim