Diagnosis. The flu can mimic numerous other ailments, most notably similar viruses that precipitate the common cold. To confirm a flu diagnosis, a healthcare professional will perform a physical examination to potentially rule out other causes. In certain instances, diagnostic tools such as the rapid influenza diagnostic, which involves a sample being taken from a patient’s nose or throat and studied for flu pathogens might be performed. However, in many cases, a physician may reach a flu diagnosis based upon the symptoms the patient presents.