There is conflicting information about whether or not these flies that look like bees are important pollinators in the garden. Some sources say they eat pollen, while other say they don't. Like bees though, they do feed on nectar and can transfer pollen from one flower to another. When I first started gardening outdoors I mistook these flies for bees. I've since learned how to tell them apart from real bees and now don't waste time chasing them around the garden with the camera.
Here are three characteristics I use to tell bees apart from flies:
The first clue is that these flies "hover" while bees don't. They way they fly is very different from bees.
The second clue is the eyes.
Flies have large eyes set on the top and front of their heads while bees have them on the side of the head. Bees have long, thin antennae while flies have stubby or no visible antennas. The real bee in the photo above is on the right while the fly that looks like a bee is on the left.
The third clue I look for to tell bees apart from the bee mimics is the wings. Bees have four wings, two per side, while flies only have two, one on each side. Bees will hold their wings over their back while flies hold splayed out to the sides like in the image above. The bee is on the right and the fly is on the left.
These are just the three characteristics I use to identify these flies. They also have different looking legs and mouth parts and unlike bees they don't carry pollen sacks. See the photos in the post on the urban bees in my garden to see for yourself.
Batesian mimicry is pretty cool, eh?