Responsive design is a regular requirement in web development now. Even if you use a theme or framework that’s meant to be responsive, things can go wrong. It’s important to do responsive design testing regularly to insure your site is behaving properly on different screens. Ideally you’d want to try on different devices and screen sizes, but while you’re working you can also test using these resources that were collected on the CSS Designers & Developers group on LinkedIn.
Firefox Responsive Design View is a Firefox developer tool you can install that has some nice options for responsive design testing and seeing a page in different sizes and perspectives.
mobiletest.me is an easy way to perform responsive design testing and see just how bad your site looks on most phones. This makes me want to make some adjustments right now.
Responsinator will let you do your responsive design testing by showing you how your site looks on lots of devices at once. Not as precise as mobiletest.me, but you see more at once. You might also feel unncessarily comfortable after seeing your site on iPads and other devices.
Screenfly has a lot of configuration options for your responsive design testing, letting you see your site on different devices, resolutions and perspecitves. It’s an interesting way to really look into how a site will look on every device you can imagine.
(This was originally on InteractiveQA.com)