Police typically use three methods of determining whether a driver has had too much to drink to be driving (intoxicated), or is under the influence of drugs:
Observation. A police officer will pull you over if he notices that you are driving erratically -- swerving, speeding, failing to stop, or even driving too slowly. Of course, you may have a good explanation for your driving (tiredness, for example), but an officer is unlikely to buy your story if he smells alcohol on your breath, or notices slurred words or unsteady movements.
Sobriety tests. If an officer suspects that you are under the influence, he will probably ask you to get out of the car and perform a series of balance and speech tests, such as standing on one leg, walking a straight line heel-to-toe, or reciting a line of letters or numbers. The officer will look closely at your eyes, checking for pupil enlargement or constriction, which can be evidence of intoxication, and will judge your ability to follow exact instructions. If you fail these tests, the officer may arrest you or ask you to take a chemical test.
Blood-alcohol level. The amount of alcohol in your body is understood by measuring the amount of alcohol in your blood. This measurement can be taken directly by drawing a sample of your blood, or it can be calculated by applying a mathematical formula to the amount of alcohol in your breath or urine. Some states give you a choice of whether to take a breath, blood, or urine test -- others do not. If you test at or above .08 % blood-alcohol concentration, you are presumed to be driving under the influence, unless you can convince a judge or jury that your judgment was not impaired and you were not driving dangerously. Defense attorneys often question the validity of the conversion formula when driver's alcohol levels are based on breath or urine tests.