So, you think you want to show, and you want to buy a good dog that can win it's fair share.

HOW DO WE EVALUATE A PUP SO THAT YOU KNOW IT IS TRULY GOING TO BE A SHOW DOG THAT WILL BE ABLE TO WIN.

* To find the right breeder
Ask how many champions they have made over the years. 
Ask if the pup has a champion mother or father, or better, both parents as champions.
Ask what they will do to help you get started and how they can help through the dog's show career as a mentor. In addition to this, the breeder should test for vWD and have a history of healthy dogs. You may have to sign a sales agreement that guarantees you will show the dog.
This is the norm.

  *Any puppy that is younger than about 6 months is just a show prospect.
A 12 week old puppy is going through so many changes, that it is not easy to predict any outcome, let alone determine ratio.  If it is a breeder's pup, and they have been breeding AND showing for many years successfully, they may be able to give some idea of promise, but still that is all it is....a possibility.  I think that it is better to wait for more maturity. 

SO WHAT SHOULD THE DOG BE LIKE TO BE A GOOD SHOW DOG?

* FIRST IS CONFORMATION OR BONE STRUCTURE.
  The standard explains well the relationship of the bones which are the framework of the dog.  This is going to be fairly evident by 6 mo. You can find the standard explained at this link: http://clubs.akc.org/stca/stcastd.htm

 You are thinking: "I've heard it's hard to tell their potential at a young age.  Hard, but not impossible at taking a guess, right?  I worry that maybe her parents' backs are too long."

  My opinion is that if you have an experienced show breeder with many champions from that line, you may be able to trust that breeder's instincts. Otherwise, IMPOSSIBLE, and if you get a 14 week old that turns out to be a show dog, that is luck, not expertise on your part. 
  If you worry that the parents have long backs, DO NOT BUY THE PUP. Length of back is usually genetic.

*  SECOND IS ATTITUDE:
 A puppy may be comfortable at home, but what about noisy dog shows? Generally, a puppy that is 6 mo or older has been to show training class and is lead trained.  The best youngster will not flinch with loud noise, will walk with head held proud, and will say to the world "look at me"

* THIRD IS MOVEMENT:
By the time a pup is 6 months, the bones are largely in place and movement is generally displayed as it will be when the dog is adult. The legs should be straight, the elbows in, and a good reach and drive with front and back legs will be obvious. Also, the legs should move parallel to each other, and not wobble from side to side, like a washing machine.
 

* LASTLY, A DOG MUST HAVE  GOOD TEETH.
 You must have a correct scissors bite, a bad bite will result in a dog that will have a hard time winning even if everything else is good.
  You will not have a full adult set of teeth in place until between 5-6 months. Even with them in place, sometimes the bite can go at a later age, but it is not common.  The teeth have to be right. 

 http://www.navhda.org/dentition.html  is excellent place to have dentition explained with drawings and explanation

http://www.dentalvet.com/vets/basicdentistry/dental_anatomy.htm excellent with photos

  Total teeth in an adult canine should be 42. 

For all of these reasons, you should probably not choose a show puppy before it reaches about 6 months.  Even then there is some risk that the dog will not turn out to be a show dog. But any pup younger than 6 months is a big risk, especially to a novice. 

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