Method for retrieving managed package prefix

gistfile1.txt

    /**
     * @description determines the namespace prefix of the managed package
     */
    public static String getNamespacePrefix(DescribeFieldResult fr){
        String prefix = null;
      
        String fieldName = fr.getName();
        String localname = fr.getLocalName();
     
        Integer idx = fieldName.indexOf(localname);
        if(idx > 0){
            prefix = fieldName.substring(0,idx);
        }
        return prefix;
    }