Проверяем - отмечены ли чекбоксы

<script src="http://code.jquery.com/jquery.min.js"></script>
<input type='checkbox' id='checkbox-1' checked>
<input type='checkbox' id='checkbox-2' checked>
<input type='checkbox' id='checkbox-3'>
<input type='checkbox' id='checkbox-4'>

<script>
  console.log($('#checkbox-1').attr('checked'))
  console.log($('#checkbox-2').is(':checked'))
  console.log($('#checkbox-3').attr('checked'))
  console.log($('#checkbox-4').is(':checked'))
</script>

checked
true
undefined
false