How does a doc clinically differentiate respiratory failure from heart failure?

Not easy: A careful history, physical exam, underlying illnesses, ekg, chest xray, lab studies and possibly an echocardiogram and ct scan can all provide information that taken in aggregrate will suggest one diagnosis is predominant - though even with all this info it still occurs that both processes may be present and one may be driving the other. Only a trained physician can reasonably make sort this out.

Diagnosis...: Heart failure is one of the causes of respiratory failure. Respiratory failure occurs when the body cannot properly take in oxygen and/or eliminate carbon dioxide. A blood gas test can determine if respiratory failure is present and then the specific cause can be diagnosed via physical exam, additional tests, etc. Respiratory failure can be life-threatening so see your doctor for treatment.