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The patient’s history of recent travels, exposure
to animals or infected persons, or exposure to
the herpes simplex virus or arboviruses are important clues in diagnosis.
If the patient is unconscious or cannot speak, family members may be
asked for information about his or her recent activities.

The specific diagnostic tests that may be done depend partly on the
type of disease organism that the doctor suspects is causing the encephalitis.
Some common diagnostic tests include:

• Blood tests. Blood tests can be used to diagnose West Nile virus
  
• A neurological examination. This examination allows the doctor to
  evaluate the patient’s memory and other mental functions, sight
  and hearing, reflexes, gait and movement, mood, level of consciousness,
  and other functions of the nervous system that may
  be affected by encephalitis.
• Spinal tap. In a spinal tap, a small sample of spinal fluid is withdrawn
  through a needle inserted into the lower back. It can be
  tested for evidence of a bacterial or fungal infection. In some cases,
  a spinal tap can also be used to diagnose HSV encephalitis.
• Electroencephalogram (EEG). An EEG is a test that measures patterns
  of electrical activity in the brain. HSV encephalitis produces
  a telltale abnormal EEG pattern.
  
• Imaging studies of the brain. Computed tomography (CT) scans
  and magnetic resonance imaging (MRI) are the types of imaging
  studies most frequently used to diagnose encephalitis.
  
• Brain biopsy. In a brain biopsy, a surgeon removes a small sample
  of brain tissue to analyze it for the presence of HSV. This test is
  rarely done except when brain imaging or other tests for HSV do
  not provide definite results.