How to know that dynamically created script tag was executed?

I was able to get this to work by adding an ID to the script, then in the JS, manually firing the load event on that DOM element. Tested only in Chrome, will have issues in older IE according to MDN).

var head = document.getElementsByTagName('head')[0];
var script = document.createElement('script');
script.type = 'text/javascript';
script.charset = 'utf-8';
script.id = 'testing';
script.defer = true;
script.async = true;
script.onload = function () {
    console.log('The script is loaded');
}
script.text = ["console.log('This is from the script');",
               "var script = document.getElementById('testing');",
               "var event = new UIEvent('load');",
               "script.dispatchEvent(event);"].join('');
head.appendChild(script);

Fiddle

In modern browsers you could use Mutation Observer to detect changes in an element – head in this case. Something like this:

observer = new MutationObserver(function (m) {
    // This will be fired
});
observer.observe(document.head, {childList: true});

Unfortenately this doesn't work in IE < 11, but it seems that onload is fired in IE, so you can use it for IEs.

A live demo at jsFiddle.