It all depends on what specific category of narcotic drugs we are talking about. If it is powder: -mephedrone (alcohol / 2nd) has a special consistency that is significantly different from other things, you can still see how the will dissolve / heat up, first of all in appearance, sometimes the costs of preparation / lack of cleaning are striking, as well as in the smell (if the person really understands), you can still observe how it will dissolve , ideally there should be no insoluble impurities If this is some kind of gash, then by homogeneity / hardness Etc.
It depends on what we are talking about. Often, nothing but "try". To distinguish you need a chemical. analysis. It is possible that by the smell, and even then it is rare. You cannot tell what the water tastes and quality, if it is just clear water (of course, you will distinguish water from a puddle, but even when the cocaine is black, you suspect something). It's about the same here.