A Drug Recognition Expert (DRE) is a law enforcement officer trained to identify people whose driving is impaired by drugs other than, or in addition to, alcohol. The 12 step procedure they follow is called a Drug Influence Evaluation (DIE), to determine which category of drugs is causing the driver to be impaired.
If a DRE determines that a driver is impaired, they will look for indications of the drugs by the common perceivable effects the drugs have on the human body. There are seven categories of classifications a DRE is looking for, including: Central Nervous System (CNS) Depressants, CNS Stimulants, Dissociative Anesthetics, Cannabis, Hallucinogens, Inhalants, and Narcotics.
Truthfully, the "expert" part of Drug Recognition Expert is a misnomer, these are people who are trained to fill out a chart based upon what they see. The chart is supposed to tell them what substance is present. The DIE is a guess! It is subjective and easily manipulated. DREs are not doctors, they have not nurses - in fact, they have little to no medical training at all. The DIE is nowhere near 100% accurate and there are many ways to challenge the DRE's testimony.