I used to create a new Xcode project from template just to check how our new icon looks on a real device. There should be a better way. And today I found Ikonica.
Ikonica has a pair of OSX and iOS apps. Both are free. Fantastic!
Launch Ikonica on the Mac. You'll see a pairing screen with PIN.
With Ikonica on iPhone, you have the choice between previewing icons in your Dropbox/Camera Roll and pairing a Mac.
Choose "Pair Device" and enter the PIN on your Mac.
Now you have the icon shown on the screen.
To preview a new icon, drag it to Ikonica on the menu bar, which is the dark square box in the image below between Dash and CloudApp.
In seconds, the preview icon on the iOS app will be updated.