Adult Masked Booby can be separated from other black and white boobies by the entirely dark secondaries and dark tail. At close range, the yellow legs, feet, and bill and the dark mask are diagnostic.
Immature boobies are very difficult to identify.
Brown and Red-footed have darker bellies. Blue-footed has a white rump patch and a more horizontal demarcation between the dark upper breast and the white belly. Northern Gannets tend to lack the white collar but at a distance may be inseparable.