Dont assume as a KC breeder all is good.
Find all the health tests that should be done on your breed
Ask to see contract of sale - kc breeders should all have one
Ask why she does not have buyers lined up - good breeders would usually have a waiting list so I would be worried that she has all of the puppies available at 10 weeks,
Ask re worming and vaccination of mum and puppies
Is this the first breeding - if not contact owners that have previous pups from other litters
Speak to or see the Dad
See Mum with the litter
Ask to see her socialisation plan - if she has keep the puppies for 10 weeks she should be using one - I would be looking for accurate documentation on this, not just random things she has done.
Pick up and handle all the puppies - if she is not happy for this walk away
Get copies of health checks carried out and research results do not just glance at them - make sure you understand them and what they mean.
They should offer a lifetime guarantee to take back the puppy - not just help you to re-home it - make sure they will personally take responsibility for the puppy for life and have documentation that states this.
Make sure they are happy to support you via phone for the dogs life if needed - especially in the early days
Make sure the puppy has been raised in a house and preferably in a situation similar to yours eg with children or noise etc - not kept out in a kennel.
If she is local ask if she is happy for you to get a recommendation from her vet.
You should be offered free insurance for 4 - 6 weeks from date of purchase.
Try to look at more than one litter so you can compare
Expect to be grilled by the breeder, they may even seem rude and noisey but that is good - they must care where the dogs are going.
You can check with KC that the dogs are part of the breeder scheme.
Finally check out breed rescue to see if there is a dog available for rehoming (always worth a try)