GAP Manual: 15.2. IsCyclotomicField

IsCyclotomicField( obj )

returns true if obj is a number field record (see IsNumberField) where obj.isCyclotomicField = true, and false else.

    gap> IsCyclotomicField( CF(9) );
    true
    gap> IsCyclotomicField( NF( [ ER(-3) ] ) );
    true
    gap> IsCyclotomicField( NF( [ ER(3) ] ) );
    false