How do the Police know I’m Driving Under the Influence (DUI)?

Field Sobriety Tests and Drunk Driving Checkpoints

Researchers have developed a variety of tests that are designed to determine whether a person is likely to be intoxicated. A police officer performs these tests on suspects after the officer has stopped a person on suspicion of drunk driving. These tests allow an officer to observe a suspect's balance, physical ability, attention level, or other factors that the officer may use to determine whether the suspect is impaired. Officers often record a suspect's performance of these tests, and this practice generally has been upheld on appeal.
In several states, authorities have set up checkpoints where officers can question drivers in an effort to catch drunk drivers. These checkpoints are often set up during holidays when people are more likely to drink, such as New Year's Eve. Courts in the majority of states have upheld these checkpoints against challenges that these checkpoints are unconstitutional.

Refusing a Chemical Test: "Implied Consent" Laws

All states have "implied consent" laws that require vehicle drivers to submit to some form of chemical test, such as breath, blood, or urine testing, if suspected of DUI. The logic behind such laws is that, by assuming the privilege of driving a vehicle on state roads and highways, drivers have effectively given their consent to DUI testing when a police officer reasonably believes the driver is under the influence of alcohol or drugs. If a driver refuses to submit to such testing, implied consent laws carry penalties such as mandatory suspension of a driver's license, usually for six months to a year. Often, license sanctions for test refusal are more harsh than those imposed after DUI test failure. In most states a driver's refusal to submit to a chemical test may be used to enhance the penalties imposed if he or she is eventually convicted for DUI.

"Per Se" and "Zero Tolerance" DUI Laws

All states have DUI laws that deem "per se intoxicated" any driver with a blood-alcohol concentration (BAC) above a set limit (now .08 in all states). This means that drivers with a BAC at or above .08 are intoxicated in the eyes of the law, and no additional proof of driving impairment is necessary.
All states also carry "zero tolerance" laws that target drivers under the legal drinking age. These laws penalize persons under 21 for operating a vehicle with any trace of alcohol in their systems (a BAC above 0.0), or with negligible BAC levels such as .01 or .02.
Keep in mind that a driver may still be arrested and convicted for DUI without proof of "per se" intoxication, when other evidence of impaired driving is shown. For example, a driver with a .06 BAC level can be found guilty of DUI if an arresting law enforcement officer testifies that he observed the driver's vehicle swerving badly, and that the driver exhibited both slurred speech and severe inattention during questioning after a vehicle stop.