There are some pretty complex testing tools for responsive designs out there. I even see people constantly resizing their browser window using on-screen rulers. The easiest approach to me is just a simple page with a bunch of iframes, like Matt Kersley’s test page. Because I always ended up refreshing the whole page rather than the URL bar on that page, I saved the file locally and tweaked it a little.

This was inspired by, and based on @lensco's excellent Simple Responsive Design Test Page. It lets you view any webpage in multiple screen sizes, simulating the viewport of different devices. After getting such a positive response to my original post, I thought I'd expand on it a little. Since people are obviously targeting different device screen sizes with their projects, the form below now lets you generate a custom bookmarklet that displays only those device sizes you're interested in.