If you want to maximize the user experience, it’s imperative to test your prototypes in the native environments. Ask yourself these questions. Are all the design elements arranged in a comfortable layout? Is it easy to reach all elements with your fingers? Is it still easy if you’re only using one hand? Does your hand block important elements as you hold the phone in the palm of your hand? ... etc.
Answers to these and many other questions often cannot be easily answered while looking at the static images on your display. You can put this work together using special prototyping softwares and services or by using css3 and html5 capabilities, depends on your preferences and knowledge. At this point it’s important that you really polish all the touch areas - the user shouldn’t need to aim at targets as well as the interface shouldn’t interpret user’s touch falsely.