ok so there is good coke and bad coke. Good coke will be white, pure white, and fluffy, and when it breaks it does so as a soft white powder would, rather than like something hard or jagged and not really crunchy or crusty unless it gets wet. like he said put some on your tongue or gums and see how it feels. good coke makes ur teeth feel numb if u put it on ur gums, the stronger it is the more tingly numb your teeth become. good coke has a bitter taste to it and tastes a lot like novacaine you get at the dentist office.
bad coke will be almost sour rather than bitter, solid hard and break into wierd sticky chunky shit you can't just pulverize into a fine powder with some ease, and the numbing effect may last a very short amount of time.