how to find out if XMLHttpRequest.send() worked

XMLHttpRequest provides the ability to listen to various events that can occur while the request is being processed. This includes periodic progress notifications, error notifications, and so forth.

So:

function sendFile() {
   var form = $('#fileform').get(0);
   var formData = new FormData(form);
   var file = $('#fselect').get(0).files[0]
   var xhr = new XMLHttpRequest();
   formData.append('myfile', file);
   xhr.open('POST', 'uploadfile/', false);
   xhr.addEventListener("load", transferComplete);
   xhr.addEventListener("error", transferFailed);
  }

    function transferComplete(evt) {
        console.log("The transfer is complete.");
        // Do something
    }

    function transferFailed(evt) {
        console.log("An error occurred while transferring the file.");
        // Do something
    }

You can read more about Using XMLHttpRequest.

Something like the following code should do the job:

    xmlhttp.onreadystatechange = function() {
      if (xmlhttp.readyState === 4) {
        var response = JSON.parse(xmlhttp.responseText);
          if (xmlhttp.status === 200) {
             console.log('successful');
          } else {
             console.log('failed');
          }
      }
    }

XMLHttpRequest objects contain the status and readyState properties, which you can test in the xhr.onreadystatechange event to check if your request was successful.