In iPhone App,
while running the App on device How to detect the screen resolution of the device on which App is running?
CGRect screenBounds = [[UIScreen mainScreen] bounds];
That will give you the entire screen's resolution in points, so it would most typically be 320x480 for iPhones. Even though the iPhone4 has a much larger screen size iOS still gives back 320x480 instead of 640x960. This is mostly because of older applications breaking.
CGFloat screenScale = [[UIScreen mainScreen] scale];
This will give you the scale of the screen. For all devices that do not have Retina Displays this will return a 1.0f, while Retina Display devices will give a 2.0f and the iPhone 6 Plus (Retina HD) will give a 3.0f.
Now if you want to get the pixel width & height of the iOS device screen you just need to do one simple thing.
CGSize screenSize = CGSizeMake(screenBounds.size.width * screenScale, screenBounds.size.height * screenScale);
By multiplying by the screen's scale you get the actual pixel resolution.
A good read on the difference between points and pixels in iOS can be read here.