This is awesome, much better than Responsivator.
Stick this in in a bookmark:
javascript:void((function(){function e(e,t){var n=document.createElement('script');n.type='text/javascript';if(n.readyState){n.onreadystatechange=function(){if(n.readyState=='loaded'||n.readyState=='complete'){n.onreadystatechange=null;t()}}}else{n.onload=function(){t()}}n.src=e;document.getElementsByTagName('head')[0].appendChild(n)}e('http://breakpointtester.com/assets/js/breakpoint.js',function(){var e=document;e.write('<!DOCTYPE html>'+'<html>'+'<head>'+'<meta charset="UTF-8">'+'<title>Breakpoint Test - '+e.title+'</title>'+'<link rel="stylesheet" href="http://breakpointtester.com/assets/css/bookmark.css">'+'<script src="http://breakpointtester.com/assets/js/bookmark.js"></script>'+'</head>'+'<body data-url="'+e.URL+'">'+'<header id="topHeader" class="clearfix">'+'<a href="#"></a>'+'</section>'+'<section id="bpCount">'+'<div id="count">00</div>'+'<div id="countText"><span>BREAKPOINTS FOUND</span></div>'+'</section>'+mqUniqueBP+'</header>'+'<section id="qcWWW">'+'<ul id="qcWW"></ul>'+'</section>'+'</body>'+'</html>');});})());
I find that the simulator tools built into FF and Chrome tend to be more reliable these days b/c they will actually send UA info, so a lot of times to get good results you need to change not just screen size, but simulated device and hit F5 (I'm thinking less for my own stuff, than for other sites that serve differently to different UAs, esp if a site is using screen width.