This should be default XC as it seems 99.9% of customers (even some developers) are confused with XC OPC
Open /skin/common_files/modules/One_Page_Checkout/ajax.checkout.js and find
Code:
this.cbutton.addClass('inactive').bind('click', function(e) {
inside ajax.widgets.checkout.obj.prototype.disableCheckou tButton = function() {....} function
and immediately after add
Code:
// added by CFL Systems for message if customer is not logged in / no customer info
xAlert("YOUR MESSAGE HERE.");
This will popup the message if customer tries to click on "place order" button before cart is ready for checkout