Female sunbirds typically resemble each other closely while the metallic sheen of the males makes them easy to identify.  The Plain Sunbird Anthreptes simplex takes a bit more practice.  The male bird only has metallic purple on the forehead to show while the female seems equally drab and lacking the purple forehead.  Further examination of the female will reveal that while the rest of her plumage remains a drab olive-green, she has a greyish throat to distinguish herself from the other female sunbird species.

Plain Sunbird female

Plain Sunbird male