Something like this will show you the ids in the logger...
var form = FormApp.getActiveForm();
var items = form.getItems();
for (var i in items) {
Logger.log(items[i].getTitle() + ': ' + items[i].getId());
}
To see the IDs manually in Chrome >> View Form >> right-click on the question Label >> Inspect element >> look for the for="entry_XXXXXXX"
attribute. That 'xxxxxxx' will be the id.