Or a Struct/Class that provides this method and perhaps other methods to query the runtime environment. That seems more Swifty to me and better than adding free functions.
Using a hashmark to indicate something that doesn't resemble the C pre-processor doesn't seem that sensible, although I guess we're all used to it by now.
The Struct/Class would resemble UIKit's classes like UIDevice, UIScreen etc. that are Singletons and model aspects of the environment.