How can you tell a baby's blood type (like rh)?

Type and screen. All donated blood are also tested for ABO and rh groups, along with the presence of red blood cell antibodies. This is known as a type and screen. Typing of blood the ABO and rh status. The sample is then screened for any alloantibodies that may react with blood.
See below. 1. Wait until the bay is born, then test blood. 2. Guess based on parents ABO type. I would go for #1. Good luck.