[MacRuby-devel] How to test if you're running MacRuby vs. Ruby?

Is there a way to test if code is running in MacRuby rather than Ruby? I'm developing and testing some code using plain old Ruby 1.8.7 in TextMate, then integrating it into an Xcode project, so I need to be able to "comment out" some lines which are Cocoa-dependent when testing in plain Ruby. Paul Howson