GAP Manual: 6.1. IsField

IsField( D )

IsField returns true if the object D is a field and false otherwise.

More precisely IsField tests whether D is a field record (see Field Records). So, for example, a matrix group may in fact be a field, yet IsField would return false. 

    gap> IsField( GaloisField(16) );
    true
    gap> IsField( CyclotomicField(9) );
    true
    gap> IsField( rec( isDomain := true, isField := true ) );
    true    # it is possible to fool 'IsField'
    gap> IsField( AsRing( Rationals ) );
    false    # though this ring is, as a set, still 'Rationals'
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