It's not doable to check whether the lookup has completed or not using JavaScript. One workaround maybe check how long it usually takes for the request http://[formserver]/Forms/form/autopopulate?formId=x using browser developer tool then set the field to be readonly during that period of time. For example, I found my lookup rule take usually 3 seconds to finish, then I use following code to set the dropdown readonly for 3 seconds:

$(document).ready(function () {

    $('.dropdown select').attr('readonly',true);  
  setTimeout(function(){
     $('.dropdown select').attr('readonly',false);         
  },3000);
  
});