They should be aware of the major breed problems. Every breed has some. These are problems that occur in the breed with more or less frequency that all decent breeders try to eradicate. The breeder should know their own pedigrees well. They should know the faults their own lines may carry.
All lines have some faults.
They should be willing to put in writing what they will do if your puppy develops any of these problems. Ask for proof of any testing the breeder has claimed to have done. Ask to see the OFA papers to prove their dogs have been x-rayed free of Hip Dysplasia AND Elbow Dysplasia. Be aware that a majority of vets do not know what dysplastic hips actually look like on x-ray. What they may feel looks normal, the OFA would consider dysplastic.