Create CMYK swatch, create rectangle, select rect, fill with new swatch and run code:

if (activeDocument.selectedItems[0].style.fill.color){
Dialog.alert('Filled');
}else{
Dialog.alert('without Fill');
}

Next step: select rect and change new swatch (turn on Global option).
returns 'without Fill', why?
Same 'problem' occurs with SPOT color.