JSFiddle - React, Tailwind, and code Playground

HTML

<iframe id="ifr" src="http://example.com"></iframe>
<iframe id="ifr2" src="about:blank"></iframe>

JavaScript

function checkIframe( ifr ) {
    var key = (+new Date) + "" + Math.random();

    try {
        var global = ifr.contentWindow;
        global[key] = "asd";
        return global[key] === "asd";
    }
    catch(e){
        return false;
    }
}

$(function() {
    alert( checkIframe( $("#ifr")[0] ) +""+ checkIframe( $("#ifr2")[0] ) );
});