If checked:
$( "SELECTOR" ).attr( "checked" ) // Returns ‘true’ if present on the element, returns undefined if not present
$( "SELECTOR" ).prop( "checked" ) // Returns true if checked, false if unchecked.
$( "SELECTOR" ).is( ":checked" ) // Returns true if checked, false if unchecked.
Get the checked val:
$( "SELECTOR:checked" ).val()
Get the checked val numbers:
$( "SELECTOR:checked" ).length
Check or uncheck checkbox
$( "SELECTOR" ).prop( "disabled", false );
$( "SELECTOR" ).prop( "checked", true );