The code below shows how to filter the hidden fields.
var dg = $('#dg');
var fields = dg.datagrid('getColumnFields',true).concat(dg.datagrid('getColumnFields',false));
var hiddenFields = $.grep(fields, function(field){
var opts = dg.datagrid('getColumnOption', field);
return opts.hidden;
})
console.log(hiddenFields)