How do you tell if a cough is due to lung congestion or just sinus drainage?

Physician Eval... Often, your doctor can tell the difference by examining you, including listening to your lungs with a stethoscope while you breathe deeply. If your lungs are congested, there will be abnormal sounds over your lung fields. With sinus congestion, your lungs will sound normal (unless bronchospasm related to your sinuses is present..If so, wheezing will be heard!).
Many causes poss. There can be many causes of cough: sinus infection, allergy, bronchitis, pneumonia, acid reflux and more. It is hard to answer your question without a thorough examination and detailed history. If your cough is severe or persists an abnormally long time, see your md.