It is often quite difficult to differentiate a non-specific symptom like "cough" since many things can cause the symptom. Only by listening to breath sounds, reviewing the patient's current symptoms, patient/family medical history, and with diagnostic testing can the most well-educated physician decide whether a "cough is a cough" or from something much more serious. A previous answer stated a cough from asthma is "from the weather" but that is not true; asthma's coughing can be triggered by a wide variety of lung irritants or even by becoming too hot physically. As well, the previous answer stated that the cough of lung cancer is "from smoking" but millions of lung cancer patients never smoked one cigarette in their entire lives!
Many asthma patients "cough" instead of "wheeze". It can be a sudden, hard coughing fit that occurs fast, or an asthmatic can have small short coughing all day long, for weeks or months. If the person has sinus drainage as well as asthma, coughing may confuse the medical picture-- is the person coughing because the lungs feel tight, or because the sinus drainage is triggering the cough reflex?
In lung cancer, some patients develop a chronic slight cough early in the disease. The only early symptom may be an irritating cough. The cough occurs because a tumor, even a tiny one, irritates normal lung tissue and the body tries to "clear" (cough out) what is obstructing air flow. As cancer progresses, the cough may become productive-- the person may have mucous come up into the mouth or that the person spits out. A productive cough in lung cancer often has blood-tinges-- meaning anything from a pinkish color to bright red blood streaks. But, a person may have an occasional blood-streaked mucous even without cancer because hard coughing can break small blood vessels.
So, without looking at complete medical information and tests (X-Ray, MRI or CAT scan), no one can be certain a cough is from cancer or whether it is from asthma.