Determine the gender of the bumblebee. Female bumblebees have smaller, more delicate antennae, sometimes with a bend. Male bumblebees have thicker antennae, usually longer, without a bend. Female bumblebees have a pointed abdomen and males have a rounded abdomen. Watch the behavior of the bumblebee -- females collect pollen and males do not.
Check the hind legs of the bumblebee, looking for a pollen basket. Female bumblebees in the "Bombus" genus have a pollen basket in the upper segment (the tibia) of their hind legs. Neither male Bombus bumblebees nor female or male "Psithyrus" bumblebees have a pollen basket for collecting pollen.
Count the wings -- bumblebees in the Bombus genus have two pairs of transparent wings. One pair is larger than the other pair.
Note the coloration of the head, thorax and abdomen after determining the genus. Within the Bombus genus, many different species exist with a variety of different striping and color patterns.