I'm in the very early learning stage with breeding and showing myself as I've only been at this four years. I will share what I've learned.
My friend, who has been doing this many years says that in the first 24 hours, she does an evaluation on the head. There are things she looks for with ear set, etc. that she sees then. We will see how true this is as she whelped my two who are now three weeks old.
In the limited experience I've had with raising pups, I've been able to pick some who were the same from birth to adulthood. In a couple other situations I've been wrong because the ugliest pup in the first few weeks, and again at around ten to twelve weeks turned out to be the best. I've also seen other pups that went on to finish who were just down right ugly as a pup. One girl I know who is the "niece" to one of my boys would not have been picked by me even at four or five months, yet, she finished her championship with four majors and only showed five times to get it. By the time she was ready for the ring at ten months, she was, and still is one of the most beautiful Maltese I've seen. Of the three I have here that I'm preparing for the ring, the least impressive one is now the better of the two boys. I was going to pet him out up until he was four months old.
I like to see a pup stacked on the table with its hair wet. Hair can do a lot to cover flaws. Then, if it looks good like this, it should look great with the proper grooming. I learned this the hard way, after I decided against showing one and cut her hair. Now, I could kick myself for not keeping her in coat.