You can call JoglNewtWindow.getNewtWindow().isRealized(). The field "_inited" isn't exposed in the public API because it makes more sense to use a render queue or to call invoke(boolean, GLRunnable) once to be sure the initialization is complete.
I wanted to look at my source code to give you an example but Sourceforge is partially broken today, I'm fed up, I will end up by hosting everything on my own server that just works.