我发现答案并不包括模拟器的特殊情况。

#define IS_WIDESCREEN ( [ [ UIScreen mainScreen ] bounds ].size.height == 568  )
#define IS_IPHONE ([[ [ UIDevice currentDevice ] model ] rangeOfString:@"iPhone"].location != NSNotFound)
#define IS_IPAD ([[ [ UIDevice currentDevice ] model ] rangeOfString:@"iPad"].location != NSNotFound)
#define IS_IPHONE_5 ( IS_IPHONE && IS_WIDESCREEN )

用于检测所有版本的iPhone和iPad设备。

#define IS_IPAD (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
#define IS_IPHONE (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone)
#define IS_IPHONE_5 (IS_IPHONE && [[UIScreen mainScreen] bounds].size.height == 568.0)
#define IS_IPHONE_6 (IS_IPHONE && [[UIScreen mainScreen] bounds].size.height == 667.0)
#define IS_IPHONE_6_PLUS (IS_IPHONE && [[UIScreen mainScreen] bounds].size.height == 736.0)
#define IS_RETINA ([[UIScreen mainScreen] scale] == 2.0) 

我用hfossli的答案翻译给了Swift

let IS_IPAD = UIDevice.currentDevice().userInterfaceIdiom == .Pad
let IS_IPHONE = UIDevice.currentDevice().userInterfaceIdiom == .Phone
let IS_RETINA = UIScreen.mainScreen().scale >= 2.0

let SCREEN_WIDTH = UIScreen.mainScreen().bounds.size.width
let SCREEN_HEIGHT = UIScreen.mainScreen().bounds.size.height
let SCREEN_MAX_LENGTH = max(SCREEN_WIDTH, SCREEN_HEIGHT)
let SCREEN_MIN_LENGTH = min(SCREEN_WIDTH, SCREEN_HEIGHT)

let IS_IPHONE_4_OR_LESS = (IS_IPHONE && SCREEN_MAX_LENGTH < 568.0)
let IS_IPHONE_5 = (IS_IPHONE && SCREEN_MAX_LENGTH == 568.0)
let IS_IPHONE_6 = (IS_IPHONE && SCREEN_MAX_LENGTH == 667.0)
let IS_IPHONE_6P = (IS_IPHONE && SCREEN_MAX_LENGTH == 736.0)

我们现在需要考虑iPhone 6和6Plus的屏幕尺寸。以下是最新的答案

if(UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone)
{
    //its iPhone. Find out which one?

    CGSize result = [[UIScreen mainScreen] bounds].size;
    if(result.height == 480)
    {
        // iPhone Classic
    }
    else if(result.height == 568)
    {
        // iPhone 5
    }
    else if(result.height == 667)
    {
        // iPhone 6
    }
   else if(result.height == 736)
    {
        // iPhone 6 Plus
    }
}
else
{
     //its iPad
}

一些有用的信息

iPhone 6 Plus   736x414 points  2208x1242 pixels    3x scale    1920x1080 physical pixels   401 physical ppi    5.5"
iPhone 6        667x375 points  1334x750 pixels     2x scale    1334x750 physical pixels    326 physical ppi    4.7"
iPhone 5        568x320 points  1136x640 pixels     2x scale    1136x640 physical pixels    326 physical ppi    4.0"
iPhone 4        480x320 points  960x640 pixels      2x scale    960x640 physical pixels     326 physical ppi    3.5"
iPhone 3GS      480x320 points  480x320 pixels      1x scale    480x320 physical pixels     163 physical ppi    3.5"

借用Samrat Mazumdar的回答，这里有一个估算设备屏幕大小的简单方法。它适用于最新的设备，但在未来的设备上可能会失败(正如所有猜测的方法一样)。如果设备正在镜像，它也会混淆(返回设备的屏幕大小，而不是镜像的屏幕大小)

#define SCREEN_SIZE_IPHONE_CLASSIC 3.5
#define SCREEN_SIZE_IPHONE_TALL 4.0
#define SCREEN_SIZE_IPAD_CLASSIC 9.7

+ (CGFloat)screenPhysicalSize
{
    if(UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone)
    {
        CGSize result = [[UIScreen mainScreen] bounds].size;
        if (result.height < 500)
            return SCREEN_SIZE_IPHONE_CLASSIC;  // iPhone 4S / 4th Gen iPod Touch or earlier
        else
            return SCREEN_SIZE_IPHONE_TALL;  // iPhone 5
    }
    else
    {
        return SCREEN_SIZE_IPAD_CLASSIC; // iPad
    }
} 

非常简单的解决方法

if(UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone)
{
    CGSize result = [[UIScreen mainScreen] bounds].size;
    if(result.height == 480)
    {
        // iPhone Classic
    }
    if(result.height == 568)
    {
        // iPhone 5
    }
}