Field sobriety tests...
When an officer has his audio recorder on and verbally say's, " Your eyes are watery and bloodshot.", That is entered into evidence just because he said it, whether it's true or not. Without a breathalizer (in the case of alcohol) this statement cannot stand up as viable evidence in court. But, they sure will use it to make the arrest. Conversely, any cop can say, "You're high on pot." and it will take a blood test to determine if the drug was used in the past month. It cannot determine if the individual was impaired at the time of the arrest. Police departments say that all officers are trained to determine if someone is in fact impaired. I contend that they are, for the most part, trained to make a case.