Bookmarklet - simply show the view-port width to make it easy to see what media-query is being used.

bookmarklet.js

javascript:((function(){var e=document.createElement("div");e.style.position="fixed";e.style.height="50";e.style.width="300";e.style.margin="-25px -150px 0 0";e.id="unitseven-size";e.style.top="50%";e.style.left="50%";e.style.zIndex="1000";e.style.padding="5px 10px 5px 10px";e.style.backgroundColor="#000";e.style.color="#fff";e.innerHTML="Size Here";var t=document.getElementsByTagName("body")[0];t.appendChild(e);var n=document.getElementById("unitseven-size");n.innerHTML=document.documentElement.clientWidth;window.onresize=function(e){n.innerHTML=document.documentElement.clientWidth}})())();

Responsinator.js

/* Responsinator Bookmarklet - simply show the viewport width to make it easy to see what media-query is being applied.
*/
(function(){
	var el=document.createElement('div');
	el.style.position='fixed';
	el.style.height='50';
	el.style.width='300';
	el.style.margin='-25px -150px 0 0';
	el.id='unitseven-size';
	el.style.top='50%';
	el.style.left='50%';
	el.style.zIndex = '1000';
	el.style.padding='5px 10px 5px 10px';
	el.style.backgroundColor='#000';
	el.style.color="#fff";
	el.innerHTML="Size Here";
	var b=document.getElementsByTagName('body')[0];
	b.appendChild(el);
	var d=document.getElementById('unitseven-size');
	d.innerHTML = document.documentElement.clientWidth;
	window.onresize = function(event) {
		d.innerHTML = document.documentElement.clientWidth;
	};
})();