Most people say it is a crap shoot and I say no it is not. Picking good pups can work but does not necessarily mean you are picking the best one.
I usually do not pick the biggest nor the smallest.
I start testing the pups starting at about 7 weeks and continue until 10 months to a year of age. However, I am always watching and studying the pups from the time they are born and I handle them daily and make sure they know who I am from day one.
I first get them to like raw beef liver and stage winding situations once they are crazy about the liver. Then I hide the small pieces of liver where they can wind them and then I turn them loose and I always make note of who does what and how. I always use hunger to my advantage.
I set drags for them and see who trails. Starting at 8 weeks or younger
I do not keep hyper pups.
I like calm and laid back pups that pay attention to their surroundings. These pups might be laid back but when it's time to play they play hard but they get bored with this and go off exploring. These pups like being with you but the call to range out is stronger.
At 14 weeks I cull down to 4 or 5 pups if I am keeping 2.
At 12 weeks or so I take them to the woods and turn them loose and see who rolls out. A me too dog is a plus at this stage. Once I know who rolls out The next outing I leave that one at home and see who rolls out. I don't pay much attention to the pups so that they can focus on rolling out if they will. I make sure that they are fresh out the kennel when I do this test so I know the pups are eager and energetic.
At 4 months I try to find a shoat so they can bay it. Only short periods of baying not to burn them out and it keeps them wanting more.
at 6 months I cull down to 3 or 4 pups.
At 10 months or so I will have 3 at most.
At 1 year I will be down to 2 pups.
It all depends on the strain within a breed of dogs or the breed of dogs because not all strains or breeds develop at the same rate or age.
It is better if you can keep the whole litter and test them but usually not feasible for the average breeder due to funds or lack of space and or time.
Doing it this way usually produced hard hunting dogs that could find a hog...winding, free casting, hunting ahead of the 4 wheeler or rigging out of a boat.