Officers are trained to look for specific indicators of intoxication when they make a stop for suspicion of a DUI. At the very least, one of these indicators is mentioned in an officer's DUI arrest report to give basis for a DUI arrest. These factors are outlined in the Standardized Field Sobriety Test (SFST) Manual and are outlined in three phases below.

Just like an officer is trained to document the reason for the stop, he is also documents his observation of the events surrounding the vehicular stop. This time frame begins when the officer activates his lights to signal the car to pull over and the time that the car comes to a stop.

The SFST Manual outlines six cues that are evidence of a DUI: (1) an attempt to flee; (2) no response, (3) slow response; (4) an abrupt swerve; (5) a sudden stop; and (6) striking the curb or another object.

Usually, none of these indicators are present in a case, so a defense attorney will ask the officer about the stopping sequence so that he will testify that none of the indicators were present.

The next observation period is the officer's approach to the vehicle. This includes the officer's initial observation of the driver's physical characteristics. In particular, officers look for bloodshot or droopy eyes, a flushed face, slurred speech, and whether there is an odor of intoxicants. Officers also observe whether the driver has difficulty retrieving his license, such as whether he had difficultly locating the license or whether he fumbled with the license while handing it over to the officer.

If the driver admits to consuming alcohol during this stage of the stop, an officer may give weight to this evidence. Because the officer will not likely follow the admission to drinking with any questions, then the defense attorney may be able to combine an insufficient investigation with ongoing observations that are consistent with sobriety and will have an effect on the fact finder. A defense attorney will ask the officer follow-up questions about this conversation, including whether the driver informed the officer of the time period, or whether the driver was able to understand and answered appropriately.

Finally, officers observe the driver during an exit sequence. The SFST Manual enumerates specific behaviors that the officer is trained to look for at this stage of the stop. Some of those behaviors are that the driver: shows angry/unusual reactions, cannot follow instructions, cannot open the car door, leaves the vehicle in gear, leans against the vehicle, or keeps hands on the vehicle for balance.
Some ways to defeat these observations at trial is to get the officer to admit that the driver never put his hand on the vehicle as he walked, followed instructions about where to go, never resisted, walked in a straight line, and never stumbled or tripped.

The goal here is to leave the fact finder with a lack of credible evidence of impairment for these three sequences. By balancing the officer's evidence of alleged impairment with evidence of normal sober behavior will create a reasonable doubt in the fact finder's mind and possibly lead to an acquittal or reduction of charges.

For more information, see: 
THE DWI DETECTION AND STANDARDIZED FIELD SOBRIETY TESTING MANUAL by Stephen L. Jones