When the day for picking a puppy arrives, I go through four evaluation steps before making my final decision:
1. Tail position
Is the tail tucked between the pup's legs? This indicates the pup is timid, submissive and likely harder to train. If the tail is held high above the pup's back it indicates the pup is dominant and will also present training problems, albeit a different set of difficulties from the low-tail dog.
You want to see the tail held in a position between tucked and above the pup's back, which means the pup is neither overly submissive nor overly dominant and likely is both biddable and trainable without the issues associated with the aforementioned less desirable positions.
Has the pup been exposed to other dogs and people or just left in the pen alone? Discuss this with the breeder. You want a dog that plays well with others for a variety of reasons.
If the pup comes to me and actively seeks and maintains contact, this is indicative of a pup that should receive high marks in biddability and trainability.
Does the pup readily retrieve a thrown object such as a tennis ball or duck wing? You want a retriever that already has retrieving in his blood. If a pup has this, then you're in business. He should want to fetch up anything you throw in front of him.
4. Lip Licking
If a pup licks his lips when praised for obeying a command or making a short retrieve, that's the nail in the coffin. This shows me the pup understands my commands. If a dog is distracted/constantly looking everywhere but at me, training is much more difficult.
While there are many variables in the selection process, these are the factors I look for when selecting from a litter, with one caveat: they can never be all-inclusive.
However, in every case, if you exercise due diligence in your selection of the sire and dam of the litter and have a puppy selection procedure in mind, your chances of obtaining an excellent and healthy gun dog that will be by your side for many years will be very high.