One of the field tests typically performed by police officers is called a horizontal gaze nystagmus test. In this test, the officer asks you to face forward with your head still then follow a pen or other object as he moves it from side to side.

Assuming that you have no disease or condition that would cause your eyes to twitch or dart, a sober person will follow the moving pen smoothly. A person who has consumer alcohol will not be able to follow the object smoothly and his eyes will jerk and twitch as he tries to keep his focus.

I found this video on YouTube that demonstrates a horizontal gaze nystagmus test.