A more accurate check for retina in iOS. (Assumes all future devices are retina)

NSString+Formatting.h

@interface NSString (Formatting)

- (NSString*)stringWithOnlyLetters;
- (NSString*)stringWithOnlyNumbers;

@end

NSString+Formatting.m

#import "NSString+Formatting.h"

@implementation NSString (Formatting)

- (NSString*)stringWithOnlyLetters {
    NSCharacterSet *numSet = [[NSCharacterSet letterCharacterSet] invertedSet];
    return [[self componentsSeparatedByCharactersInSet:numSet] componentsJoinedByString:@""];
}

- (NSString*)stringWithOnlyNumbers {
    NSCharacterSet* letters = [[NSCharacterSet characterSetWithCharactersInString:@"0123456789"] invertedSet];
    return [[self componentsSeparatedByCharactersInSet:letters] componentsJoinedByString:@""];
}

@end

RetinaDisplayMethods.h

/*! @attention Not accurate on the simulator, and will not be accurate on future devices that use retina resources but actually have a non retina display*/
extern bool hasRetinaDisplay(void);
extern bool usesRetinaResources(void);

RetinaDisplayMethods.m

#import "RetinaDisplayMethods.h"
#import <NSString+Formatting.h>
#import <sys/utsname.h>

bool hasRetinaDisplay() {
    struct utsname systemInfo;
    uname(&systemInfo);
    
    NSString* code = [NSString stringWithCString:systemInfo.machine encoding:NSUTF8StringEncoding];
    NSString* deviceType = [code stringWithOnlyLetters];
    NSString* deviceVersion = [code stringWithOnlyNumbers];
    int version = [deviceVersion intValue];
    
    if ([deviceType isEqualToString:@"iPad"])
    {
        if (version < 30)
            return false;
    }
    
    if ([deviceType isEqualToString:@"iPod"])
    {
        if (version < 50)
            return false;
    }
    
    if ([deviceType isEqualToString:@"iPhone"])
    {
        if (version < 40)
            return false;
    }
    
    return usesRetinaResources();
}

bool usesRetinaResources() {
    return ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)] &&
            ([UIScreen mainScreen].scale == 2.0));
}