warn user if all checkboxes are unchecked

I have a form with a series of checkboxes. I want to warn the user, after they hit submit, if ALL of the checkboxes are unchecked. I'm using the following code to report all of the values of the checkboxes:

$('[id^=leg_rider]').filter(':checked');

This seems to work. However, when I try to check to see if the returned object is empty, it doesn't seem to work. This is what I'm trying:

        $("#set_pref_submit").click(function() {
        var legchecked = $('[id^=leg_rider]').filter(':checked');
        if (!legchecked){ alert("no riders")};
    });

Any suggestions are appreciated. Thanks!