"Evidence-gathering technology for drugs is not as advanced in terms of ease of use and noninvasiveness as it is for alcohol. Until recently, no simple test police officers could administer to obtain an indication of drug use similar to the preliminary breath test for alcohol has been available. Rather, samples of urine or blood typically must be sent away for laboratory analysis to determine the presence of drugs and their quantification. Screening tests using urine, which can be used by officers in the police station, have been field tested by NHTSA. The technology is also developing for using saliva, sweat, and hair samples to detect drug use (Hersch, Crouch, & Cook, 2000).
"As said earlier, NHTSA has funded the Drug Evaluation and Classification (DEC) program, which equips specially trained officers, known as Drug Recognition Experts (DREs), to observe and record behavioral evidence of drug use to assess potential drug impairment among persons suspected of drug-impaired driving, and guide chemical testing and expert testimony for DUID trials. Currently, more than 40 States have officially adopted DEC programs to train DRE personnel."
Lacey, John, Brainard, Katharine, and Snitow, Samantha. (2010). Drug Per Se Laws: A Review of Their Use in States. (DOT HS 811 317). Washington, DC: National Highway Traffic Safety Administration, pp. 5-6.