California is cracking down on driving while under the influence, or DUI cases. With their strict penalties and careful monitoring of the roads, some drivers may find it beneficial to know exactly what they're looking for when they try to determine whether or not a driver may be drunk.
The first way is through field sobriety tests. FieldSobrietyTests.org offers different examples of real sobriety tests that are used by officers. They include both standardized tests, which will hold up better in a court due to their regulations, and non-standardized tests. It's up to the police to decide which test or tests will be administered. Generally speaking, this is just the first step. It's possible for an officer to arrest someone if they fail a field sobriety test, but these are commonly followed up by breath tests instead.
After an officer has reason to believe that someone has been drinking, either through the use of field sobriety tests or through suspicious behavior, they may issue a breathalyzer test. Forbes states that breathalyzer tests work by measuring the amount of alcohol in a person's blood through their oxygen. Their blood alcohol content level (BAC) is what's looked at to determine if someone is "over the legal limit" or not.
Generally speaking, these are the two ways that police will determine if someone is drunk while driving. It isn't enough to simply suspect someone is drunk because of the way they're looking or acting, so these specific steps must usually be taken first.