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$( "SELECTOR" ).attr( "checked" )  // Returns ‘true’ if present on the element, returns undefined if not present
2$( "SELECTOR" ).prop( "checked" ) // Returns true if checked, false if unchecked.
3$( "SELECTOR" ).is( ":checked" ) // Returns true if checked, false if unchecked.

1// jQuery version 1.6 or above use:  
2$("#myRadioID").prop("checked", true);
3 
4//jQuery versions below 1.6 use:
5$("#myRadioID").attr('checked', 'checked');

1if ($('#check_id').is(":checked"))
2{
3  // it is checked
4}

1$(".checkbox").change(function() {
2    if(this.checked) {
3        //Do stuff
4    }
5});
6