GAP Manual: 15.1. IsNumberField

IsNumberField( obj )

returns true if obj is a field record (see IsField, Field Records) of a field of characteristic zero where F.generators is a list of cyclotomics (see chapter Cyclotomics), and false else.

    gap> IsNumberField( CF(9) ); IsNumberField( NF( [ ER(3) ] ) );
    true
    true
    gap> IsNumberField( GF( 2 ) );
    false