Experience
I have bred one litter, and sold exactly one puppy. That puppy went to a multi-MACh agility person, one who didn't even want a puppy
I knew when this puppy was young that she needed a job. She was not hyper, she was not all over the place wild. But there were some signs. First, she LOVES toys, will carry anything in her mouth, regardless of size (sticks, giant tennis balls; she will find a way). She did not seem to have any mental boundaries about what she should and should not do. She was the first puppy to wander around my house looking at everything, first on the sofa, first on the contact trainer, etc. Third, she always wants to play, with anything. None of my dogs are sound sensitive, so I didn't expect my puppies to be either. My house is not a quiet one, and I live close to a major freeway.
So when Barbara came to look at the puppy, she ran her through a couple of exercises. First, just basic toy drive. We pulled out a fleece braided toy, and the puppy played with that for half an hour with Barbara. Then we broke out the wobble board, and she very quickly had her standing with all four feet on the board, balanced in the middle for a few seconds. That puppy was totally into her, could not have cared less that I was there. (Interestingly enough, her sister was shown by a handler at the National last year, and also could not have cared less that I was there, he was her whole world)
So I think that I would look for a dog who looks like they are thinking. They don't have to be wild and crazy, just deliberate. The one that has no problems leaping from the couch to the coffee table and surveying their domain. I want them to carefully watch the agility dogs, not bark and lunge at the dogs in the ring.
People talk about structure, but I think balance is important. My parents have a rescue dog who is HORRIBLY structured, completely straight front and rear, and short backed. But because his front are rear are balanced, the dog still moves decently at 13, and finished his HT last summer. His topline is still pretty level, there is no twisting or bouncing, and he is still the first to the tennis ball.
Lots to think about.
Rachael
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The Munchkins: Marque, Katie (the big Collie Munchkin), Liesl, River, Evie