How do bees sense sugar?

A bee's organ of smell is its antennae, and its sense of smell is many more times as sensitive as our own. It has taste receptors on its antenna, particlarly near the tips, on the mouthparts and within the mouth, and some on the legs. (Imagine being able to taste things with your legs!).
Bees are attracted to flowers by the smell and the colours, and natural nectar has a smell from the flower from which it came. Pure refined sugar doesn't have a smell, and it is possible to put a feeder on top of a beehive containing pure sugar syrup and the bees won't know it is there. They will find it eventually because there are always bees walking around all surfaces of the hive provided the weather is warm enough.