Knowing How to Communicate With the Breeders will help you in finding the best breeder for you.
The environment plays such a large role in what the puppy becomes that any testing done at such a young age is only useful as an evaluation for what parts of socialization the breeder has missed. At 49 days the pups are at their most malleable stage. An example of this are adult dog behaviors that are not innate but are learned. Good working dogs are developed from tiny puppies. Their early experience and training are carefully tailored to what they will be doing as an adult. Hence the need to make a continuous training plan for your puppy. Keep in ind puppies should stay with their litter at least 7 days past the 49 day mark to give them more time with littermates to learn dog behavior.
Recent research suggests that looking at puppies chest whorls, paw preference (right or left) and preferred eyes may also play a role in predicting dogs that will be successful as guide dogs.
Dogs become accustomed to cues from vertical people.
If the pup’s future handler is in a wheelchair, make sure breeder gives the cues while sitting as well as standing. For someone on bed rest, the pup needs to be acclimatized to the handler giving cues in that position.
Consider the specific breed or mixes if breeders do or require spay or neuter at a young age (with the breeder or under a year of age). Many breeds are negatively affected when they are spayed under 18 months or neutered under a year. Affects can be both behavior and medical/structural. This can affect the ability of the dog to be a service dog. Check this article and chart to see if your breed is affected.
If you can’t find a breeder locally, it is well worth the time and cost (think of it as an investment) to travel to find one that is suitable. It is possible to do most pre-screening by phone and web cam to make sure the pup is a good prospect before going.
Be very hesitant to buy a pup without seeing the parents first. What your interpretation of what a dog is may be very different than what the breeder’s interpretation is. You could hire someone knowledgeable about puppies to do an onsite visit for you after you talk to the breeder and get their permission but it is still second-hand information.
If you are seriously interested in one breed, join a national or regional breed club and see what other breeders say about a prospective breeder. They know the strength and weaknesses of their competition. If you talk to them, you’ll find out pretty quickly what breeder focusses on.