Good coke is usually white/off-white in color, it tastes 'icey' and will numb your mouth. (Though other local anaesthetics may also be present.)

All the heroin I have encountered was brownish in color, though it may also come as a white or yellow powder. It was bitter-tasting and sort of reminiscent how a vitamin tablet would taste.

Though it probably isn't true in every case, the packaging of the substances was also different. The heroin obtained in my area is enclosed in squares of wax paper, the empty wrappers litter the streets. The coke and crack always came tied in a plastic bag.