Detect a click in a cross domain iframe

index.html

<iframe id="iframe" src="//example.com"></iframe>
<div id="message"></div>

index.js

var monitor = setInterval(function(){
    var elem = document.activeElement;
    if(elem && elem.tagName == 'IFRAME'){
		message.innerHTML = 'Clicked';
        clearInterval(monitor);
    }
}, 100);

style.css

iframe {
    width: 500px;
    height: 300px;
}