Similar species
On a sitting female, black on the chin and throat makes a dark arrow pointing into the white breast, and on a flying bird, the border seems perpendicular to the flight direction. On a female Great Frigatebird, the white breast points into the black, almost reaching the bill. Up close, look for different color of eye ring
Adult male has different color gloss on mantle when seen up close, and lacks the paler line on upper wing usually seen on Great Frigatebird. If visible, leg color should be diagnostic.
Juvenile Great Frigatebird is usually yellow/orange/tawny on head and upper breast, but the lack of that is not a completely safe field mark for Magnificent.