Limelight Responsive is a wonderful online tool to test if your website is responsive. You can preview your website in 15 different devices and one of the greatest features is that you can see if you have any errors in your code (based on common mistakes). Another great feature is that Limelight will also help you with meta viewports as it auto detects them.
Responsive webdesign is really popular among designers and developers nowadays. But when designing a website how you can be sure it will look great at all screen sizes? It is good to have a set of tool to help you in designing a responsive website.