Detect Retina Display

In order to detect the Retina display reliably on all iOS devices, you need to check if the device is running iOS4+ and if the [UIScreen mainScreen].scale property is equal to 2.0. You CANNOT assume a device is running iOS4+ if the scale property exists, as the iPad 3.2 also contains this property.

On an iPad running iOS3.2, scale will return 1.0 in 1x mode, and 2.0 in 2x mode -- even though we know that device does not contain a Retina display. Apple changed this behavior in iOS4.2 for the iPad: it returns 1.0 in both 1x and 2x modes. You can test this yourself in the simulator.

I test for the -displayLinkWithTarget:selector: method on the main screen which exists in iOS4.x but not iOS3.2, and then check the screen's scale:

if ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)] &&    ([UIScreen mainScreen].scale == 2.0)) {  // Retina display} else {  // non-Retina display}

@sickp's answer is correct. Just to make things easier, add this line into your Shared.pch file:

#define IS_RETINA ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)] && ([UIScreen mainScreen].scale >= 2.0))

Then in any file you can just do:

if(IS_RETINA){   // etc..}

+(BOOL)iPhoneRetina{    return ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)] && ([UIScreen mainScreen].scale == 2.0))?1:0;}