Below is a list of things I consider when looking for a future agility teammate. As you read the list, you will probably notice that there is a lot of educated guessing that has to take place with a puppy, remember you have other options.
Selecting an older puppy or rescue sometimes gives you a much better picture of who that particular dog is, and whether they might be a good agility prospect. True there is a lot of foundation work that you do with puppies, but a bright, active youngster who is keen and ready to work can make a fabulous team mate and many of the questions about temperament, motivation, reinforcement, energy, speed, health etc. are easier to assess. It often doesn’t take long at all to build the foundation you need to start your agility training in earnest.
Research - Talk to breeders and make sure they know what you are looking for - most will be very interested to match a puppy with what you want to do. Talk to people who have dogs you like and find out about their experiences in acquiring the puppy. Find out as much as you can about the breeder’s program and goals and use social media to research and track dogs in the same lines. In short educate yourself and become a partner with the breeder in making a selection.
Interact - Go meet the puppies as much as you can - I think its very important for you to get the puppy that you want. This is not what most breeders would allow or even advise. Typically, they will pick the puppy for you. This is not necessarily a bad thing- it might prevent you from picking the one that looks you in the eyes first, or has the most beguiling colors or some other irrational process. However, for me, I think puppies also have an opinion about you. As flexible as they are, puppies do demonstrate that they are naturally attracted to you or not. How loud your voice is/how exuberant your body language is and probably other things I can't describe (smell, medications...). I like to know that the puppy shows some indication that my natural energy is a good fit.
Reinforcment - In order to get a dog who will work hard to learn things that you want them to learn - and will enjoy doing that - you have to know that you have something that you can offer them that will make it worth their while. For agility - the most versatile reinforcers are tug, fetch and food - so give the puppies a chance to play tug the one that is most persistent about enjoying that game is a puppy who inherently can be easily reinforced with something you have to offer. Fetch is another great marker - take a little pinecone and throw it 2 feet - the puppy who doesn't notice - not a good choice, the puppy who takes it and runs away with it - better, but not great. the puppy who chases it, grabs it and turns toward you with it - thats the one (even if the fetch itself is very rudimentary). The puppy that is most motivated by attacking its siblings when given a chance to play with you - is a puppy that is not inherently reinforced by what you have to offer it - you can still build it, but much easier to start with a pup that is inclined to be interested in you.
Personality - For agility, curious, bold puppies will naturally find agility pretty easy. Ones who bounce back well from a startling experience will tend to be able to naturally deal with the competition environment better. These pups will also tend to be most coordinated just because they will have explored lots of stuff already and will have learned how to use their body better than less intrepid explorers. Every temperament type will have pros and cons - so pick a puppy that will require training in areas that you enjoy - for example, if you like building confidence in a puppy, don’t worry too much about a more thoughtful pup - but if you find that tedious - then look harder at the more outgoing pups - realizing that your training is going to have to emphasize impulse control instead.
Fit - Remember that you have to live with this dog. Despite all of the "intellectual" reasons to pick a puppy - you will find that dealing with their neurotic behaviors, chewed up possessions, barking etc. will be much more tolerable if you pick the puppy that you want to live with. My older dog is completely wrong for agility from an intellectual perspective - she was a total choice from the heart - and she's been a nightmare to train - but I don't mind because she was the one I wanted and she wanted to come home with me.
Gender - Everything else considered, I think this is the least pertinent factor in picking a performance puppy - it is more likely to be a factor in getting along with other household members. The only things that I have found that might influence the performance side of things is that I think females tend to mature faster than males which I think only influences the training stages - not necessarily the ultimate results. There is also the factor of dealing with spay/neuter timing and/or disruptions to your training/competing for heat cycles and if either of those is an important factor for your family.
Conformation - the original purpose of good conformation is a sound dog who can work all day. If you are in danger of getting hooked on agility - you want the most sound pup you can find with good angulation in the front, a long neck, a robust topline, balanced angulation in the rear, angled croup, and in short excellent working conformation. Show conformation tends over time to exaggerate structure - if angulation is good, more angulation will stand out even more - so make sure you focus on what will enable the dog to do agility, (particularly jump easily and happily)
Energy level - young puppies give a good indication of their energy level by the time they are ready to go to new homes. The puppy that dashes to investigate things of interest is a higher energy puppy. The puppy that trots is moderate. The puppy that walks or toddles to investigate is lower energy. Remember that drive and speed are not saying the same things. The lower energy puppy that works and works and works to get a piece of kibble out from under the couch might be high drive, but low speed. Remember that you also have to live with the puppy that you pick out!
Early Learning - Meet the parents if you can, especially mommy. Mom has a big influence over the puppy early in life. Generally a robust, well-balanced temperament in the mom will start the learning in a direction you will like. A nervous/anxious/noise sensitive/push over etc... mom, might not so much. A reputable breeder who intends their puppies to do well in performance homes will have plans to expose the puppies to safe and stimulating environments - find out what those are and monitor their progress as the puppies grow up.
Health - Do your best to make a selection with eyes wide open as to what a lifetime commitment to this puppy means. So many dogs, and often well-bred dogs have issues that can be exacerbated by the rigors of agility training and competition. Many things are treatable, operable, manageable, addressed with physical therapy etc. and some things could just become chronic conditions that you and your dog need to deal with. Be prepared to provide excellent supportive care to any puppy.