What initials would a cardiologist have after his name designating that he is a cardiologist?

Physicians are unique in that the type of medicine they practice are almost always not indicated by abbreviations. In the US, only M.D.s and D.Os are physicians. The only way to tell if a physician is a cardiologist would be those who choose to list their professional association of the American College of Cardiology. Thus, a fellow of this organization would be FACC. The name would look like Thomas Brown, M.D., FACC. Again, though, not all physicians list this. The only way to really know is to ask.