What makes a bee a bee? Turns out this is not an easy question! Bees are an epifamily (made up of 6 families) within the order Hymenoptera (with wasps, ants, and a few other related insects). They are distinguished from other Hymenoptera be the presence of branched "hairs". While these split ends are about as hard to find as you would imagine, they correspond to an important behavioral distinction. Unlike nearly all other Hymenoptera, bees feed their offspring pollen, and the branched hairs are an adaptation for collecting pollen.
Determining if an given insect is a bee or not is often challenging and there aren’t any sure fire shortcuts. The best strategy is to look at a lot of labeled pictures and develop a search image for all the possible styles of bees. Below are two grids to help get you started. Each image can be click for a larger version. Pay attention to the shape and placement of the antenna and the position and pattern of the wings.