How to differentiate between iphone4 and iphone 3

Question

I am trying to build a game for the iphone using cocos2d engine. I wanted to know how can I tell a difference whether the user is using iphone 4 or iphone 3 as I wanted to l

Answer 1

For detecting retina display on all devices including the new iPad

    +(BOOL)isRetinaDisplay {
    // since we call this alot, cache it
    static CGFloat scale = 0.0;
    if (scale == 0.0) {
        // NOTE: 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)) {
            scale = 2.0;
            return YES;
        } else {
            scale = 1.0;
            return NO;
        }   

    }
    return scale > 1.0;
}

Credit to: Adriano Paladini http://developer.appcelerator.com/question/133826/detecting-new-ipad-3-dpi-and-retina