gtk:window-is-fullscreen (window)

window -- a gtk:window widget

The boolean whether the window has a fullscreen state.

Retrieves the current fullscreen state of window. Note that since fullscreening is ultimately handled by the window manager and happens asynchronously to an application request, you should not assume the return value of this function changing immediately, or at all, as an effect of calling the gtk:window-fullscreen or gtk:window-unfullscreen functions.

If the window is not yet mapped, the value returned will whether the initial requested state is fullscreen.