Hyperventilation is breathing at a faster and/or deeper rate than the body requires for good oxygenation at the existing work level.  Normally the rate of breathing is controlled by the amount of carbon dioxide in the lungs and in the blood.  In hyperventilation carbon dioxide is blown-off and this leads to changes in the acidity of the blood which causes symptoms.  The arteries to the brain constrict, reducing the blood supply.  Pilots may notice slight  dizziness, a feeling of coldness, a sensation like a tight band around the head, and pins and needles in the hands and feet.   Paradoxically they will often feel they cannot get enough air.  Continued hyperventilation may cause unconsciousness.  Hyperventilation most commonly occurs in association with anxiety, fear, or during intense concentration on a difficult task such as performing a complicated instrument procedure.  

The symptoms, particularly the shortness of breath, are not unlike those of hypoxia, so rather than trying to make the diagnosis, the following procedure should be taken:
a) Breathe oxygen, if available, at 100% and, if on a pressure demand system, "press to test".   After 3 or 4 breaths of oxygen the symptoms will improve markedly if hypoxia is the cause.  
b) If the symptoms persist, consciously slow the rate of breathing to 10-12 breaths per minute and do not breathe deeply.  Keep the respiratory rate slow until the symptoms disappear and then resume a normal breathing pattern.  [ If below 8000 feet ASL (2500 m) hypoxia is unlikely to be the cause of the problem].