Recent research shows that flies are a surprisingly important group of pollinators. They are present in most ecosystems and they tend to circulate earlier in the season than many of our bees. Flies visit flowers to sip on nectar and end up moving pollen resulting in pollination. Some plants, like skunk cabbage, give off odors that attract the flies. Did you know that flies are the primary pollinators of chocolate?
Telling the difference between bees and flies is not difficult but does take some practice. Flies have two wings while bees have four. That is tricky to see as the insects are flying in the garden. Look at the insect's face. Bees usually have prominent antennae while a fly's antennae are shorter and not as noticeable. Fly eyes tend to dominate their faces while the compound eyes of bees are more separated. Fly bodies may have some hair but flies are not nearly as hairy as most bees. The Census Insect Counting & Identification Guide is a great resource to learn to differentiate between these two types of pollinators.