How can you tell if a newborn with blood group o has antibodies for both a and b antigen?

Blood group antibody: At birth a kid with blood group o negative will have no original antibodies to any blood group. There may be some transient anti-a or b passed along with shared maternal antibodies, but these will disappear over a few months. Specific testing might detect them, but for no real benefit. In life we can develop original anti a or b from things we eat.