if checkbox is checked

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

1if ($('#id_input').is(':checked')) { }

1function checkAddress()
2{
3    var chkBox = document.getElementById('checkAddress');
4    if (chkBox.checked)
5    {
6        // ..
7    }
8}