for window in root_win.query_tree()._data['children']:
    window_name = window.get_wm_name()
width = window.get_geometry()._data['width']
height = window.get_geometry()._data['height']
if width == screen.width_in_pixels and height == screen.height_in_pixels:
    num_of_fs += 1
print(num_of_fs)
How to tell if a full-screen application is running?