Using the scripts onload and onerror events is most likely an option here I suppose.

  
  // Set up the onload handler before appending to DOM
  tag.onload = () => {
  //Check if Google Pay exists
  if ('google' in window && window.google.payments && window.google.payments.api) {
      loadGooglePay();
    } else {
      console.error("Where response???");
    }
  };