1//using plane javascript 
2if(document.getElementById('on_or_off_checkbox').checked) {
3    //I am checked
4} 
5
6//using jQuery
7if($('#on_or_off_checkbox').is(':checked')){
8    //I am checked
9}

1$('.myCheckbox').prop('checked', true);
2$('.myCheckbox').prop('checked', false);

1$("checkbox").is(":checked")

1// Check a checkbox
2$(selector).prop('checked', true);
3// Un-check a checkbox
4$(selector).prop('checked', false);
5// Determine if checked or not
6isChecked = $(selector).prop('checked');

1if ($('#grepperRocks').is(':checked')) {
2	// this checkbox is checked
3}

1$('.myCheckbox')[0].checked = true;
2$('.myCheckbox')[0].checked = false;