what signs from his childhood does amir think of that suppor the idea that his father is also hassans father?

Signs from Amir's childhood that support the idea Babba was also Hassan's father were the fact that Babba never forgot to remember Hassan's birthday, he always referred to both Hassan and his father as family, Babba cried the day they packed up and left his home, and also the fact that Babba paid for Hassan's surgery on his harelip.