Thats a really difficult question as they are so young. Normally when pups are about 6 weeks old one says dont take the lethargic one, take the lively one. However, we were proven wrong by this when our beagles gave birth. The last to be born was tiny, fragile and at 6 weeks very quiet. However, later she became the most intelligent of them all and the naughtiest. I would say it is almost impossible to choose straight after birth unless you know you only want a girl (or boy)and there is only one of that sex. Then reserve that pup and hope for the best.