Answer: The things we know are:
Sadie, the mother, is a black labrador.
Sam, the possible father, is a yellow labrador.
Discarding things such as a scientific indagation, the breeder could look for traits in the pups.
For example, yellow is recessive against black, so if the pups are yellow, there is probable that the father is Sam, none the less, if the pups are black there is also the possibility where Sam is the father, just in this case the color genes that the pups received are the ones of the mother.
A good starting point is looking at the color, the posture, and things like that.
In the end, these things are only statistics, there is not any determination here, the only real (scientific) determinative process is a paternity test.
