A puppy is a twelve-year commitment to walks, lifting and vet visits. The questions worth asking a breeder are mostly boring on paper: health tests, temperament, how much exercise the adult will need. Ask them before the litter wins the argument.
Which health tests should a breeder show you?
Hip and elbow scores for the parents in medium and large breeds, the breed's known eye tests, and the DNA tests that apply to that breed. A breeder who has the paperwork offers it; a breeder who waves it away is telling you something.
For the specific case of white shepherds, a site on white shepherd genetics explains the e/e genotype behind the coat and which health tests, from MDR1 to degenerative myelopathy, actually matter for the lines.
Does coat colour tell you anything about health?
Sometimes, but less than folklore claims. White coats in shepherds come from a recessive masking gene, not from albinism, and the colour itself is not a disease. The conditions that do cluster with the breed sit in the health-test list, not in the pigment.
How much work will the adult be?
More than the puppy suggests. Ask what the parents' daily exercise looks like and be honest about yours. A dog whose needs match your week's real capacity is a companion; one that outpaces it is a guilt machine with a lead.
Our guide to choosing a dog when your back complains weighs the pulling question directly.
When should you walk away?
When the answers are vague, the paperwork is promised rather than shown, or the adult's exercise needs exceed your honest week. Another litter is always coming.
If the route in is a shelter rather than a breeder, the volunteering guide covers that side of the work.
Sources
For how a responsible rescue approaches rehoming and health, see Dogs Trust.