There are several strategies used in checkpoints in order to filter out drunk drivers. The police may either stop every car or use a specific pattern in order to stop only specific cars. While the officers on the ground are conducting the actual searches, there are "chase" cars stationed in hiding spots at the checkpoint to drive after any cars that attempt to evade the checkpoint altogether.
During a sobriety stop, the officer will likely walk up to the car being stopped and begin speaking with the driver. He or she may ask questions about where they are driving from and whether they have been drinking. However, the conversation is just a cover for the actual investigation being conducted while the officer and driver talk. The officer is actually searching for signs from the behavior of the driver that may indicate that they have been drinking regardless of whether they admit to it. The signs searched for at sobriety checkpoints are more personal traits as opposed to actual motor skills.
The ways that you can prevent unnecessary suspicion include, but are not limited to, the following:
Have your license and registration ready for the officers, fumbling and searching will arouse suspicion.
Speak clearly and definitively, uncertainty or slurred speech can cause the officers to suspect impairment.
Always remain calm and respectful, nervousness and agitation can result in suspicion of your sobriety.