You can tell by 98.5% surety by checking the eyes if someone is under the influence.
Doesn't have to be alcohol. Could be drugs- prescription or illegal.
Considered the most scientific of the three standardized field sobriety tests, the horizontal gaze nystagmus test measures nystagmus (the involuntary jerking of the eyeballs), which becomes more evident when a person is under the influence of alcohol and/or drugs. To administer the test, the officer will ask you to keep your head still while following a small object, such as a pen or a penlight, with your eyes.
While you are performing the test, the officer will look for three clues in each eye. These include: inability to smoothly follow the stimulus with your eyes, pronounced nystagmus when your eyes are all the way to the side, and the onset of nystagmus prior to reaching 45 degrees. If the officer sees four out of six clues, he or she can then arrest you for DWI (not quiet true- you have to complete the rest of the SFST and to a Breath test... then you can arrest.
After the Field Sobriety Test you are then taken into the Police Station for a breathalizor test. The breath test you do on scene is not valid in court- but helps establish probable cause after the SFST. If you fail the SFST but blow a .000, odds are you are under the influence of drugs. We then give a unrine test, or if the subject insists, a blood test.
Of course if you refuse any part of the STFT, the breath test or the Breathalizor you are charged with refusal to submit- the fine is equil to that of DUI. You can't get out of the refusal charge. You have to explain it to the subject- but you don't have to give them more than one chance.