First, make sure it is not a sow which has cubs. Don't be anxious to shoot, and make sure the bear is alone. Cubs quite often lag behind the sow. If a bear looks back often, or gazes at one spot repeatedly, wait for awhile to be sure there are no cubs. Always take time to carefully observe groups of two or more bears.
Bears are usually quite solitary unless they are siblings or are breeding.
The odds are good that two or three bears together may be a group of young bears or a sow with older cubs. Check the bear for size. Large bears appear blocky, have short stout legs, and their ears look small in relation to their heads. Small bears look rangy and long-legged, and the ears appear relatively large in relation to their heads. Younger bears are curious and less wary than older ones, so look closely before shooting a bear that doesn't seem too worried about your presence.