A doctor will do a physical exam and ask about their medical history. No laboratory tests diagnose separation anxiety disorder, but blood tests and other things can help rule out an illness or medication side effects.
If the doctor can't find a physical cause for the symptoms, they'll likely refer the child to a specialist who diagnoses and treats mental illnesses in children and teens. They use specially designed interview and assessment tools. A diagnosis is based on reports of the child's symptoms and their observation of the child's attitudes and behavior.