Don’t pick a dog or puppy that is hiding in the corner, he may be sick or have fear aggression. Fear aggressive dogs can be retrained with lots of patience but is that what you’re looking to do? If not trained properly a fear aggressive dog may bite the well meaning hand that tries to pet him. Pick a dog or puppy that comes to you, tail up high and wagging, ears perked and eyes looking at you. Drop a set of keys or bring a squeak toy to get the dogs attention. Is he curious to seek out the noise or is he frightened and running in the opposite direction? A dog that has his curiosity peeked but does not get overly excited is a good temperament dog, as is the dog that doesn’t run away from the noise but checks it out and acts as if it’s no big deal. Next clasp your hands around the puppy or small dog’s stomach and lift him off the ground a few inches holding him there for about forty seconds. Does he struggle to get away or does he just hang there without much objection? A puppy or dog that doesn’t fuss very much is also a good pick as it shows a calm submissive behavior. The next test is to pick up the pup or smaller dog and cradle him in your arm so that his belly is face up. For a larger dog have him lie down and roll him over on his back holding him gently there for thirty to forty seconds. Does he lay there calmly like a baby or does he struggle to get loose and turn himself over? If he lies there comfortably, you’ve got a good calm submissive dog. If he struggles and is unable to relax he may be displaying signs of dominance. A dog who feels he is dominant will take a firmer and more consistent handler. I don’t mean to dissuade anyone from getting the dog they fall in love with, I’m just saying that you will be in it for the long hall, how much time can you spend to work on such issues?
There is a good site for giving puppy temperament tests. Print off the test and take it with you when looking at a pup or older dog.