Can we have specific age?
Has she had eggs before?
Is she broody?
Has she recently mated?
Is she starting to form nest?
A very young duck-hen will have little chance if not at all to have eggs and ducklings.
Even if she has hatched some ducklings, they will have little chance of survival for a starter mom.
Now with a 2 year old duck hen or a duck hen that has been having eggs and hatching them makes them great mothers.
And I think it will be pretty obvious.
But you can't tell if duck-hens will be great mothers just by appearance.
It's all in experience.
First timers/hardly successful. Made Babies before's/ Very successful *IF she has been broody or had a lot of hatchlings before*.