How do I know what type of personality and temperament my puppy will have?

I interact with and spend a lot of time observing each litter from birth. I handle them every day and I watch them closely so I know which ones are more dominant, passive, playful, rough, affectionate, etc. I also usually see one or 2 that are the quickest learners and tend to reach all their milestones first and often lead the others into new things.
In our fall 2007 litter there was a single pup out of the 12 that always stood out from the rest. He was more curios, reasoned things out days before his littermates, and was by nature a leader. There was also a female that wanted nothing more than to cuddle and be pampered. She needed a bit of extra help when it came time to start eating puppy food because she was constantly pushed out by the more assertive pups. Both pups loved people, yet their basic personalities were very different. Only because I spent so much time with them was I able to see their place in the pack pecking order and help the correct families get the specific type of pup that best suited them.
Some breeders will tell you that’s it’s impossible to determine the basic personality type of a pup at this age… but I’ve had very positive feedback from those who’ve gotten my pups that what I saw in their pup is what they got. So I’m convinced it can be done… you simply need to spend the time with them to see it. This is one of the big reasons why I limit the number of litters I produce. I want to be able to spend this time with all of them and help to direct them into the right home where they’ll be enjoyed and spend their entire life. If their basic temperament and personality are a good fit, that is much more likely to happen.