How do they suspect symptoms of asd in a newborn?

They don't: An atrial communication, a "hole", (pfo- patent foramen ovale) is present in all newborns. An ASD (atrial septal defect), a persistent important atrial communication is generally diagnosed later. The pfo closes soon after birth usually. A heart murmur or other abnormal heart sound is heard by the pediatrician at an office visit, and the child referred to a pediatric cardiologist for diagnosis.

No symptoms: ASD is asymptomatic for years if not decades. It is picked up by auscultation or echo. Therapy is done to prevent future rhythm problems or heart failure.