They seriously need to offer a demo.
As a web designer, I know, simply shrinking the viewport of your browser does NOT produce results identical to your mobile device. I use the iOS Simulator that comes with Apple's Dev Tools all the time. It seems to be the only truly accurate representation of what your design will look like on iPhone/iPad. Having said that, switching back and forth is pain. If Duo even came close to being accurate, it could be worth it.