As flu is quite common most doctors usually do not perform specific tests in order to identify the influenza virus, but rely on the patients’ symptoms and the presence of influenza virus in the community. There are numerous types of influenza virus tests rapid (antigen detection) tests, or fluorescent antibody tests. The former can detect if you have influenza, but not the type of flu, whereas the latter can distinguish between the types of influenza virus (A and B).
According to the United States Centers for Disease Control modern influenza virus rapid diagnostic tests have the sensitivity of 70-75% and specificity of 90-95% comparing to the viral culture.
Commercially available rapid diagnosis tests can detect influenza within 15 minutes, but those will only identify the disease and not its type. Some tests are also sometimes performed to identify secondary infections.