Do all of the previously mentioned tests, but also look at the puppy’s ball/prey drive. He should actively chase a ball or rag, and play with it intently.
This ball/prey drive is of utmost importance for any working or competition dog. Without it, you’re not going to go very far. Many amateurs question how much drive a puppy can have. Many breeders with inferior litters will try to convince you that low drive puppies are normal, and that drive comes with age. This is untrue. While drive can be built with age, if you don’t have any from the start, there’s going to be nothing to build.
I’ve seen eight month old puppies with the ball drive of a cat. Puppies with high ball drive are easier to introduce to new stimuli, too. If the puppy should be temporarily traumatized by something, you can usually get him over it by turning the negative stimuli into something positive by associating it with the ball. If you don’t have a puppy with good ball drive, you can’t do this. And remember, drive can always be eliminated, but it can never be created if it isn’t already there to some extent or another.
If you are choosing a dog for a dog sport which has a protection/bite work component, be sure to pick the most dominant pup in the litter. To do this, you may need to trust the breeder to some extent, or come back at weekly intervals to see which puppy is consistently the most dominant. Generally though, if a litter is from “hard” or “dominant” lines, most of the pups in the litter will be (relative to other litters) fairly dominant. Again, this gets back to the role of genetics and it’s affect on choosing a puppy.
Above all else, remember the cardinal rule about when it comes to choosing a puppy: Use common sense!