Can I detect if screen is regular or compact from code on iOS 8?

Yes you can, UIViewControllers now have a traitCollection property which has information from the device idiom, to the current size classes and more ... Furthermore you can implement the method func traitCollectionDidChange(previousTraitCollection: UITraitCollection) to get notifications when the size class (or any trait) has changed (as when the user rotates the device on an iphone). The properties of UITraitCollection that you are looking for are horizontalSizeClass and verticalSizeClass ..Here is a reference

Hope that helps

You can also detect kind of device and its orientation using below extension:

extension UITraitCollection {    var isIpad: Bool {        return horizontalSizeClass == .regular && verticalSizeClass == .regular    }    var isIphoneLandscape: Bool {        return verticalSizeClass == .compact    }    var isIphonePortrait: Bool {        return horizontalSizeClass == .compact && verticalSizeClass == .regular    }    var isIphone: Bool {        return isIphoneLandscape || isIphonePortrait    }}

From the page you linked (emphasis added):

The UITraitCollection class is used to describe a collection of traits assigned to an object. Traits specify the size class, display scale, and idiom for a particular object. Classes that support the UITraitEnvironment protocol (such as UIViewController and UIView) own a trait collection. You can retrieve an object’s trait collection and perform actions when those traits change.

As noted in the references linked from there, you can implement the traitCollectionDidChange method in your view or view controller to find out when the size class changes.