When designing graphics for iOS and Android devices, there are quite a few resolutions to take into account, and retina displays make it quite hard to design interfaces on your PC or Mac that look pixel perfect on the high resolution display devices.
To get around this resolution issue, and for quick testing of your Photoshop (or other graphic editors) designs on any iOS or Android device, there are some software packages that you should consider checking out:
The above applications basically allow some form of editing on PC or Mac and showing the result on your mobile device in realtime. They differ in various ways, so have a look at them all and find which works best for your graphic design workflow.
Create a new instance of the file you are working on, and move it to a different monitor and do your live previewing from that second file instance. Then you can zoom, pan, etc, in your first instance, and the second instance will maintain its size and view, whilst the actual design changes are still reflected in both instances.
Photoshop workflow from a designers perspective: http://bjango.com/articles/appdesignworkflow/
This article covers many aspects of the mobile design workflow in Photoshop, including creating @2x graphic assets and slicing to really speed up design work.