jQuery Ajax checkbox state

if ($("#yourCheckboxID").is(":checked")) {      // checkbox is checked } else {    // checkbox is not checked }

will do the job.

Something like this:

<script type="text/javascript">    $(document).ready(function(){        $("input:checkbox").change(function() {             if($(this).is(":checked")) {                 $.ajax({                    url: 'on_off.aspx',                    type: 'POST',                    data: { strID:$(this).attr("id"), strState:"1" }                });            } else {                $.ajax({                    url: 'on_off.aspx',                    type: 'POST',                    data: { strID:$(this).attr("id"), strState:"0" }                });            }        });     });</script>

Combining your solution and the accepted answer by Ain:

<script type="text/javascript">  $(document).ready(function(){    var isChecked = $("input:checkbox").is(":checked") ? 1:0;     $.ajax({            url: 'on_off.aspx',            type: 'POST',            data: { strID:$("input:checkbox").attr("id"), strState:isChecked }    });          });</script>