check if checkbox is checked jquery

if ($('#check_id').is(":checked"))
{
  // it is checked
}

$(".checkbox").change(function() {
    if(this.checked) {
        //Do stuff
    }
});

if (!$("#checkboxID").is(":checked")) {
    // do something if the checkbox is NOT checked
}

var $boxes = $('input[name=thename]:checked');