It is unlawful to drive in Los Angeles under the influence of alcohol, or drugs, or both. You can be charged with driving under the influence in Los Angeles, if you are 1) intoxicated and 2) driving. Both elements have to be present. So, if you are simply intoxicated, you cannot be charged with a DUI. If you are simply driving with no intoxication, you cannot be charged with a DUI.
If you are driving under the influence of alcohol, it is easier to detect intoxication. There are visual signs, such as blood shot eyes, slurred speech, incoherent sentences and smell of alcohol. There also tests that can be administered to determine someone is under the influence of alcohol. Legislation has determined that someone who has a blood alcohol content of a .08 or over, is likely intoxicated for purposes of a DUI.
When it comes to being under the influence of drugs, it is a little more difficult. Of course, some drugs may be easier to detect than others, but there are also ways to determine whether someone is intoxicated by drugs.