In California, there are several types of sobriety tests that can be administered to help officers figure out if you are DUI or not. When the police initially stop you, they are likely to use either field sobriety tests, passive alcohol sensors or both to try to determine if you've been drinking and the degree of your impairment.

Field sobriety tests consist of a series of tasks that test balance, coordination, and the ability to juggle multiple tasks at once. The officer can administer whatever tests he or she chooses, but the most commonly used tests are the three recommended by the National Highway Traffic Safety Administration. These are the walk-and-turn, the horizontal gaze nystagmus, and the one-leg stand. You’ve probably seen some unlucky individual trying to perform these tests on the side of a highway-attempting to stand on one leg, attempting walk a straight line, or trying to follow an officer’s pen with their eyes. These tests have an accuracy rate as high as 77% for the horizontal gaze nystagmus and as low as 65% for the one-leg stand.

A passive alcohol sensor is like a breath test for alcohol that you don't have to breathe into. It detects alcohol on your breath from a short distance away from your face. Some police officers even have passive alcohol sensors hidden on their flashlights. Passive alcohol sensors are used to determine if there is probable cause to arrest you for California DUI. 

California police officers may also request that you breathe into a portable breath testing machine, called a preliminary alcohol screening test, or PAS test. These machines are not admissible in court, but a high reading on a PAS test will provide probable cause for an officer to arrest you and have you do a more accurate breath alcohol test at the station. 

Once arrested for California DUI, you have the option of providing a breath test, blood test or urine test. Most police stations are only equipped with a machine for the breath alcohol test. However, breath alcohol tests are not completely reliable. They are calibrated for an average person, and don't take into account natural variations between the amount of alcohol that shows up in people's breath and the amount that's actually present in their bloodstream. You do have the right to get your own blood sample taken for later independent analysis, and the arresting officer should advise you of this. However, there is no penalty if the police don’t tell you. Since the breath test is not always reliable, you should insist on having a blood test done and a separate sample saved to be analyzed later. 

If you refuse the test, you have violated California’s implied consent law. Basically, any time you put your key in the ignition and start your car, you are giving your consent to provide a chemical test if a police officer requests it of you. 

What about personal sobriety tests? These little contraptions claim to give you an accurate reading of your BAC, so you’ll know if it’s safe to take the wheel or not. Should you trust them? Not necessarily. Depending on the model, personal sobriety tests can sometimes produce readings that skew high or low. That’s not a problem if you’re just checking to see how drunk you are as part of a fun party game, but it is a huge problem if you’re relying on the test to determine whether or not it’s safe and legal for you to drive. If the machine reads even a couple of percentage points too low, or if you test your breath too early in the game, before all of the alcohol in your stomach has had time to reach your bloodstream, you could be in trouble. Saying that you checked your BAC before driving with a personal sobriety test will not get you out of trouble if the cops pull you over later and get a higher reading on their machine.