Check if at least one of a group of checkboxes has been checked.

oneChecked.js

(function ( $ ) {
    $.fn.oneChecked = function() {
      var checked = false;
      this.each(function() {
        if($(this).is(':checked')) {
          checked = true;
        }
      });
      return checked;
    };
}( jQuery ));

//use
jQuery('form').submit(function() {
  if(!jQuery('input[name="fizz[]"]').oneChecked()) {
    alert('Must pick one!');
    return false;
  }
});