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A good breeder can match you with the correct litter and pup fairly accurately.
I know several breeders of who prefer to pick pups for a potential client.
They know their lines.
As for knowing if they are reputable check/ask for references, health issues, pictures of their kennels, whelping areas etc. Those can give you an idea of whether they are a mill.
Regarding testing look on this site in knowledge base for the volhard temperament test. It has worked great for me.
