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

1if (!$("#checkboxID").is(":checked")) {
2    // do something if the checkbox is NOT checked
3}

1$(your_checkbox).is(':checked');

1if($('#on_or_off_checkbox').is(':checked')){
2    //I am checked
3}

1var $boxes = $('input[name=thename]:checked');