Testing Responsive Designs
Like most things IT, testing something on the actual device you want to test for is the gold standard. It's probably the only way to be sure of picking up those little quirks that could spoil the look or functionality that you are after.
With testing responsive design on a website, the same thing applies. However, sometimes you want to get a general idea of how the site behaves on different types of devices. Of course you can just reduce the browser window width on your desktop PC or Mac, but that's a bit finicky if you want to stop at specific window sizes.
There are a few great online tools that let you test your site quickly and painlessly: Here are three:
There is a good discussion of these three on Design Shack.