Testing your themes on small screens and different devices brings a new set of challenges. Again, you have a few options:
The book, WordPress Mobile Development Beginner's Guide by Rachel McCollin, published by Packt Publishing, gives you a full guide to making your theme mobile and testing across a variety of mobile browsers and devices. Find out more at http://www.packtpub.com/wordpress-mobile-web-development-beginners-guide/book.