How can the coroner know if the person died from smothering?

Death by suffocation causes certain specific changes to happen in the body. The Coroner knows what these changes are. Stand in front of a mirror and hold your breath. Look in the mirror and see what you look like as your brain is deprived oxygen. You will notice changes to your face. These changes happen in the smothering victim, too, only they get worse. Once you begin to breathe again, notice what your face looks like. You will see the changes begin to disappear, as the tissues in your face begin to get oxygen again. It isn't just your face that is affected by lack of oxygen, but your entire body. The Coroner knows what to look for.