The invalid input box has 'validatebox-invalid' class. Search all these input boxes and you will be able to get all the invalid form fields.
var fields = $('#ff').find('input[textboxName]').filter(function(){
return $(this).textbox('textbox').hasClass('validatebox-invalid');
});
fields.each(function(){
if ($(this).hasClass('datebox-f')){
// is datebox
} else if ($(this).hasClass('textbox-f')){
// is textbox
}
})