Diagnosis is generally determined by observation of evidence of facial weakness and a vesicular rash. Occasionally, a nerve conduction study may be done to determine the extent of damage to the facial nerve and potential for recovery.

An otoscope may show an inflamed ear canal and lesions on the pinna of the ear are indicative of the presence of varicella-zoster virus. Blood tests may confirm this but are not usually carried out. They may also be damage to the hearing of the affected side and the patient can report nausea and loss of balance. This can be due to damage to the eigth cranial nerve. If double vision is present the sixth cranial nerve may also have been damaged.

There is a promising laboratory technique called "PCR", which can detect very small amounts of viral DNA in the affected skin. However, it is mostly used for research.

The use of neuroimaging (pictures of the brain), particularly MRI (magnetic resonance imaging), can sometimes show inflammation of the facial nerves and determine whether the infection has spread to other nerves or the brain. A spinal tap is used in rare cases, especially when the diagnosis is not clear.