So far the only way to truly test responsiveness is to actually code it. You can mockup your screens in different sizes, but it won’t be truly responsive. I know Anima, a free Sketch plugin, has their tool for you to fake it in a browser, where it will act like a fixed grid. It’s actually pretty cool! I tried it once and found it helpful when you need to explain responsiveness to somebody who doesn’t understand. That’s the closest thing I can come up with.