There are tests that police use in the field to determine if a driver is operating his or her vehicle while intoxicated by alcohol. The king of these tests is the Breathalyzer testm which provides a way to measure the amount of alcohol in a driver's blood. Police can also use gaze tests, smell your breath and have you perform other field sobriety tests to gauge how intoxicated you are.
When it comes to drugged driving, however, police don't have such a clear way to determine if you have drugs in your system. For example, marijuana can be found in your bloodstream and urine for as many as four or five weeks following the use of the drug, but police can't use such tests to determine if you're high on marijuana right now. As for cocaine, it will stay in your system for one or two days. Again, you might have evidence of these drugs in your system but not be intoxicated at all.
Sometimes, police can use "Drug Recognition Experts," also known as DREs, to evaluate what kind of drugs are impairing you. A DRE can look closely at your eye movements, your behavior and other indicators to determine if you're under the influence of drugs and what kind of drug could be influencing you. These specially trained experts might perform tests on you after police bring you back to the police station following an arrest.
Although drugged driving may be slightly more difficult to prove, police are skilled at providing evidence in the form of what they observe and see in your behavior, in addition to video evidence of you and your driving to try and prove a drugged driving charge. As such, if you've been accused of this crime, you will want to carefully craft your criminal defense to try to avoid conviction
Source: Wolff & Sonderhouse LLP, "Drunk Driving," accessed Dec. 22, 2017