If a police officer pulls you over on suspicion of driving under the influence (DUI), he or she will likely arrive at that suspicion after talking to you. Officers often mention smelling alcohol or listening to someone slur their words as the things that tip them off that the person is intoxicated. They may then ask suspects to do field sobriety tests and arrest them if they fail. Some also use portable breath tests.
However, none of that happens until after the traffic stop. One important thing to remember is that the stop itself also has to have reasonable justification, and this has to present itself long before the officer can talk to you and claim to smell alcohol on your breath. The authorities need reasonable suspicion to stop your car initially or it may be a violation of your rights.
Reasonable suspicion does not mean stopping anyone who is out late at night. It does not mean stopping people who fit a profile, perhaps based on their age or attire. These types of random stops, where officers just hope to find a drunk driver, are illegal.
Instead, the police need to see indicators that the driver may be drunk. For instance, you could swerve in and out of your lane or straddle the white line down the center. You could be driving far too slowly or taking too long to start moving again at a green light. Of course, officers can also pull you over for breaking common traffic laws, like speeding or running a red light.
The chain of events is very important. If you are facing charges, you must know what defense options you have.