There's a pretty mature library (by me) for that called SDVersion. You can check for running device's model, screen size and many other parameters. It also supports OSX.
Example:
// Check for device model
if ([SDVersion deviceVersion] == iPhone6)
NSLog(@"You got the iPhone 6. Sweet !");
else if ([SDVersion deviceVersion] == iPhone6Plus)
NSLog(@"iPhone 6 Plus? Bigger is better!");
else if ([SDVersion deviceVersion] == iPadAir2)
NSLog(@"You own an iPad Air 2 !");
// Check for device screen size
if ([SDVersion deviceSize] == Screen4inch)
NSLog(@"Your screen is 4 inches");