U donít and you canít ---all you are sending out is a promise--- all you would be able to do is GUESS.
You can tell a little bit by the parents and the more you know of the linage the better you can GUESS.
The longer you keep the puppies the more they show the better you can GUESS.
You may have a puppy that fires up quicker or likes to chase things and hold on, you may have a puppy that allows his nose to take him places... one that looks and acts like one of its parents--- BUT at the end you just never know until they come of age--- until they go through the fire you will never know of his/her staying abilities---takes TIME--- all folks are sending out is a PROSPECT.
On a personal note if I had land I would NEVER get rid of a single puppy--- I would keep them all test and work them and then determine, but NONE would leave my yard.
IF someone is culling an entire litter or a puppy at 12 weeks then I promise you they have NO CLUE what they are doing and I would completely ignore their BS they are lieing.
Dont believe the fairytales.