Try focusing on the "big picture", not just the one symptom. What I mean is ask yourself some of the following questions-
Is she running a fever? How long has she had the cough? Does she seem short of breath or seem to be breathing really fast, even when just sitting still? Is she still playful and generally content or is she lethargic? Is the cough so bad that it's interfering with sleep? Is she eating and drinking? Peeing regularly? Answering questions like these can often give you a pretty good idea of whether or not it's time to call a doctor.
Keep in mind that a cough is usually the last symptom to completely go away. If your child has asthma or some other type of respiratory condition, you may need to call a dr. sooner because a cough is often the first sign of a potential problem. I know from personal experience that we, as parents, are often more bothered by our children's sickeness than they are. When in doubt, call your pediatrician or speak to his advice nurse and tell them your child's symptoms before making an appointment. Sometimes they can give you a home remedy and tell you what danger signs to watch for. Hope this helps!