As far as determining probable cause for a DUI stop or arrest, the standard of proof is relatively low. An officer can cite any number of minor incidents or factors, such as driving too slow or fast, breaking too often, or weaving, as reasonable suspicion to stop a driver.
Once stopped, the officer need only say that they could smell alcohol on the driver or that the driver had bloodshot eyes or responded too slowly to questions and instructions to justify the use of Standard Field Sobriety Tests (SFST) or the use of a Breathalyzer test. Formal blood or urine chemical tests take place at the police station after a person has been arrested.