I laugh when I see those scenes in the movies where a cop puts his finger in a baggie containing white power, puts some on his tongue and declares, “This is cocaine” or “This is PCP” are complete none for at least two reasons:
1. Putting an unknown substance/drug/chemical on your tongue, even a small amount, can be extremely hazardous or even fatal.
2. There is no definite way of telling what a substance/drug/chemical is by tasting it.
In real life, law enforcement officers, wearing protective gloves, test an unknown substance by mixing it with a chemical which acts with a substance, resulting in a specific color which identifies what the substance is.