The three standardized field sobriety tests are horizontal gaze nystagmus (HGN), one leg stand (OLS), and walk and turn (WAT).    HGN: checks the eyes for involuntary jerking when following a pen or similar object  OLS: checks for balance as the driver raises one foot off the ground for 30 seconds   WAT: checks for balance as the driver walks back and forth on an imaginary straight line