Experience is the best teacher for learning to spot contraband drugs and alcohol and drug intoxication in the field. Appearance, odor, context, packaging, and the reputation and demeanor of the people associated with the alcohol and/or drugs provide officers with more than enough information to develop what they believe to be probable cause for further investigation or an arrest.
Of course, "probable cause" isn't what a police officer believes to be sufficient evidence for an arrest - it's what a "reasonable person" would find sufficient. Those supposedly reasonable people don't see the world through a cop's eyes, so the cop needs a more demonstrable standard. It's here that presumptive field testing for alcohol and drugs is most useful.
TV and the movies used to show officers dipping their fingers into whatever powder they found, then tasting it before pronouncing it to be heroin or cocaine. The class on learning what drugs taste like wasn't included in my academy training, possibly because cyanide and cocaine look a lot alike. Media can be a more powerful teacher than formal training. In May 2010, a College Park (Ga.) PD police officer tested positive for cocaine after he backed his patrol car into another police vehicle. He claimed he taste-tested drugs he confiscated from suspects earlier in the day.
Presumptive drug testing is much safer, more defensible in court, and usually cheaper, if you include the cost of wrecked patrol cars and hospital bills for taste-testing cops. Most testing products marketed to law enforcement are chemical-based, self-contained, and disposable. Each test consumes a small sample of the drug and produces a color change for a positive result. There is another field testing method that consumes no sample at all, and indicates specifically what drug is present, e.g. opiate, cocaine, methamphetamine, etc.
Chemical Testing Kits
Self-contained chemical-based presumptive testing kits are available from companies like Jant Pharmacal (Accutest), BAE Systems (under the NIK and ODV labels), MMC International, and Mistral. The packaging differs, but the methodology is essentially the same.
The user inserts a small (pinhead-size) sample of the suspected drug into a plastic tube or chamber of the test unit, which is then sealed. The user then breaks a glass ampoule inside the tube or chamber, releasing the testing reagent, and shakes the tester to mix the sample and reagent. A specific color change indicates what drug, if any, is present in the sample. The user then books the self-contained tester into evidence, or more commonly, throws it away.
Variations of this process use multiple glass ampoules inside the test unit. If there is no color change after breaking the first ampoule, the user can break a second one inside the same kit to test for another substance.
A typical testing kit contains tubes or pouches for multiple drugs. Kits from Mistral have the user wipe a surface with a trace of the substance, using a special test paper. The user then sprays the paper with a reagent and looks for a color change.
Which vendor or product you choose or buy has a lot to do with the nature of the drug testing you do in the field. If you see mainly methamphetamine or cocaine on your stops, then it makes more sense to acquire test kits specific for that drug. A negative test won't tell you what drug (or baking soda) you have on hand, but you won't have a lot of unused tests going to waste, either.
If you deal with multiple drugs of abuse, and/or your officers aren't able to make an educated guess as to which drugs they have, then it's better to have a multi-drug test kit with a protocol for narrowing down the type of drug at hand. This can be an expensive approach. Each test tube or pouch costs from $1.25 to $7, depending on form factor, vendor, and drug type, and some protocols require up to seven tests before the user can say either "this is <drug>" or "this is not <the drugs we're equipped to test for>."
Another factor to consider in your buying decision is shelf life and tolerance to environmental conditions. Most test kits have a shelf life from one to two years. Kits past their expiration date may yield spurious results, making their use worse than no test at all.
The reagents in the kits can be sensitive to temperature extremes and bright light. A kit carried in the trunk of a patrol car through a hot summer may undergo chemical changes to the reagents inside. A test conducted in freezing conditions might not be reliable, as most vendors recommend tests be conducted with everything at room temperature.
Each agency's situation is unique. Analyze the variables of common drugs encountered, sophistication of your officers, number of tests conducted, cost, temperature, and other environmental extremes so that you'll be prepared with the right questions for potential suppliers.